'Rainbow' was a US series of tentative strategic plans for war with any of several nations, but primarily Germany and Japan, and with or without allies (1938/October 1941).

During the years between the end of World War I in November 1918 and the start of World War II in September 1939, there was a small but continuous effort in Washington, DC, to formulate, assess and improve the war plans on which the US Army and US Navy would undertake operations in the event that the USA had once again to go to war. I was the task of the officers involved in this process to undertake a study of all the international situations which might emerge and compel the US government to resort to the use of armed force, and then to propose the courses of action that the services should be ready to take. From tune to time the Department of War and the Department of the Navy approved one of these studies as a specific war plan to serve as the guide for the special planning and preparations of their staffs and operating commands. Moreover, several of the war plans were prepared jointly and therefore came to be approved by both departments for joint operations by the US Army and US Navy.

Thus, during the 1920s and 1930s, the US Joint Army and Navy Board developed a number of 'Color-coded War Plans' to outline potential US strategies for any or several possible (or rather hypothetical) war scenarios. The plans were developed by the Joint Planning Committee, which later became the Joint Chiefs-of-Staff Committee, and were officially withdrawn in 1939 in favour of five altogether more relevant 'Rainbow' plans developed to meet the threat of a two-ocean war against multiple enemies.

The use of a colour coding system for US war planning stemmed from the wishes of the US Army and US Navy to use the same symbols in their plans. At the end of 1904, the Joint Board adopted a system of colours, symbols, and abbreviated names to represent countries, and many war plans became known by the colour of the country to which they were relevant. This was a convention which survived to the end of World War II. As the convention of using colours became accepted, some colours were eventually reused: 'War Plan Gray', for instance, originally referred to Italy but eventually became a plan for the capture and/or occupation of the Azores island group.

In all the plans, it should be noted, the USA was always 'Blue'.

The plan which was the subject of the greatest work and subsequent development was 'War Plan Orange', which took the form of a series of contingency plans for a war with Japan alone: this plan was outlined in 1919 and officially adopted in 1924. 'War Plan Orange' was thus the basis, in part at least, for the war actually waged against Japan in World War II, and included a vast economic blockade from mainland China and the plans for interning the Japanese-American population of the Hawaiian islands group.

'War Plan Red' was the plan for war against the UK and Canada, while the plans for operations in different British territories had war plans of different shades of red: the UK was 'Red', Canada 'Crimson', India 'Ruby', Australia 'Scarlet' and New Zealand 'Garnet', although it should be noted that Ireland, at the time a free state within the British empire, was 'Emerald'. The plan was kept updated as late as the 1930s.

'War Plan Black' envisaged a war with Germany. The best-known version of 'War Plan Black' was conceived as a contingency plan during World War I in case France fell and the Germans attempted to seize French possessions in the Caribbean or launch an attack on the eastern seaboard of the USA.

Many of the colour-coded war plans were extremely unlikely given the state of international relations in the 1920s, and were entirely in keeping with the military planning of other nations. Junior officers were often allocated the task of updating each of the plans as a means of keeping them trained and busy, and this was especially the case with 'War Plan Crimson' against Canada.

Interestingly, although the US had fought its most recent war against Germany and would fight another with that country in little more than 20 years, intense domestic pressure developed against 'War Plan Black' when it became known that this plan was being further developed. Isolationists opposed all consideration of commitment in any future European conflict, and this may have led the US Army to focus on more speculative scenarios in its planning exercises.

During the 1910s, relations between Mexico and the USA were often volatile, and in 1912 President William H. Taft considered the despatch of an expeditionary force to protect foreign-owned property from damage during the Mexican Revolution. This was the stimulus for the creation and further development of 'War Plan Green'. In 1916, US troops under General John J. Pershing entered Mexico in search of Pancho Villa, whose army had attacked Columbus, New Mexico, and at an earlier time US naval forces had bombarded and seized the Mexican port of Veracruz, and forced Victoriano Huerta to resign the presidency. In 1917, British intelligence intercepted a telegram from the German foreign ministry to its embassy in Mexico City offering an alliance against the USA and assistance in the Mexican reconquest of the south-western states of the USA. Released to US newspapers, this 'Zimmermann telegram' helped to turn US opinion against Germany and further worsened US/Mexican relations. Relations with Mexico remained tense into the 1920s and 1930s.

Between the US Civil War (1861/65) and World War I (1914/18), the US military had frequently intervened in the affairs of Latin American countries, including Colombia/Panama, Haiti, Cuba and Nicaragua. This policy continued during the 1920s and 1930s, and parts of 'War Plan Gray' and 'War Plan Purple', though never officially activated, were used.

Some of the plans were expanded to include war against a coalition of hostile powers. The most detailed of these was 'War Plan Red-Orange', which envisaged a two-front war against the Anglo-Japanese alliance, which in fact expired in 1924 and was nor renewed. This was the contingency which most worried US planners, as it entailed a two-ocean war against major naval powers. Theories developed in war-gaming 'War Plan Red-Orange' were useful during World War II, when the USA engaged the Axis in both the Atlantic and Pacific simultaneously.

The USA was all too aware of the fact that Japan had used the opportunity afforded by its involvement on the Allied side in World War I to establish itself as a major power and as a strategic rival to the USA in the Pacific Ocean. After World War I, most US officials and planners considered a war with Japan to be highly likely in the medium term, although the US thinking was curtailed after the Japanese civilian government temporarily halted the nation’s programme of military expansion, a process which was reversed in 1931 with the rise of strong militarism and military-dominated government in Japan. The resulting 'War Plan Orange' was the longest and most detailed of the colour-coded plans.

However, following the events in Europe in 1938 and 1939 (the German Anschluss with Austria, the Munich Agreement yielding the Sudetenland area of Czechoslovakia to German, the German occupation of the rest of Czechoslovakia, and the Molotov-Ribbentrop Pact between Germany and the USSR), US war planners realised that the USA now faced the possibility of war on multiple fronts against a coalition of enemies. To that end, the Joint Planning Board developed a new series of 'Rainbow' plans, the term being a play on the earlier colour-coded plans.

During these years between the world wars, the national policy of the USA was deeply influenced by popular beliefs about national security, which had in common the concept that the USA should not enter into military alliances or maintain military forces capable of offensive operations. National policy therefore provided only a narrow basis and limited scope for military planning. During the 1920s the USA was a leading light in the promulgation of international agreements, prompted by the unprecedented human and economic cost of World War I, to limit naval construction and also to 'outlaw' war. In the 1930s the USA experimented with the use of diplomatic and economic sanctions to discourage military aggression, and with legislation which was intended to keep the USA of European and Asiatic wars. As international tension increased during the 1930s, however, President Franklin D. Roosevelt became increasingly concerned about the USA’s diplomatic and military weaknesses, but it was not until the summer of 1939, with Japanese forces already deeply committed in China and Germany on the verge of adding a military component to its existing diplomatic efforts to secure European hegemony, that Roosevelt took official note of the joint war plans of the US Army and US Navy. The planners had just finished a study of the situations in which the USA might enter a war begun by Germany and Japan. By the outbreak of World War II in September 1939, the US Army and US Navy were deeply involved on their first strategic plan for coalition warfare, on the hypothesis that the USA would join the European colonial powers (the UK and the Netherlands) in defending their common interests in the western basic of the Pacific Ocean against Japanese aggression.

The strategy of a war in the Pacific against Japan was the only part of US military and naval planning which possessed a lengthy and continuous history. Since the early part of the 20^th century it had been clear to the US government that should it ever oppose Japanese imperial aims without the support of the UK and Russia, might be faced with the prospect of having to choose between withdrawal from the Far Fast and war with Japan.

After World War I the US Army and US Navy paid ever increasing attention to this contingency as a consequence of a resurgence of Japanese imperialism, the exhaustion of Russia and its alienation from the Western world after the Bolshevik revolution had turned Russia into the USSR, the disarmament of the USA, and the withdrawal of the USA from its temporarily close association with the European colonial powers. In the Pacific the Japanese had strengthened their position early in World War I by taking the Mariana, Caroline and Marshall island groups, and Japanese control of these strategically located island groups was confirmed in 1920 by its receipt of a mandate from the League of Nations. After the 1^st Washington Naval Treaty of 1922, the USA started to lag behind Japan in the construction of new naval vessels.

The US Army and US Navy watched with increasing concern during the 1930s as Japan acquired control of Manchuria to become the puppet kingdom of Manchukuo, seized strategic points along the north coast of China and forbade access to the mandated islands. The Japanese government began to act with growing confidence, in the belief that the USA, USSR and European colonial powers were not likely to take concerted action against its expansion: in 1933 this confidence led to the Japanese withdrawal from the League of Nations after this toothless body had refused to recognize the Japanese puppet regime in Manchukuo. Once it had done this and received no response from the rest of the world, the Japanese government served notice, in accordance with the 1922 treaty terms, of its intention to withdraw from the 1922 and 1930 naval limitation agreements, both of which accordingly expired in 1936.

By the mid-1930s US military and naval planners had come to the conclusion that Japan could be defeated only in a long and inevitably costly war, in which the Philippine islands group would be lost early in the conflict and in which US offensive operations would have to take the form of a 'progressive movement' through the mandated islands, beginning with the Marshall and Caroline island groups, to establish a secure line of communications to the western basin of the Pacific. The planners thus faced the question of whether the makers of national policy meant to run the risk and incur the obligation of engaging in such a war. The Department of State had not relaxed its opposition to Japanese expansion on the Asiatic continent, and this opposition, for which there was much popular support, involved the threat of war.

After the Philippine Independence Act had passed successfully through the Congress in 1934, the Department of War came to believe ever more strongly that the USA should not run the risk or incur the obligation of fighting the Japanese in the western part of the Pacific. When the question finally came up in the autumn of 1931, the US Army planners took the position that the USA should no longer remain liable for a fruitless attempt to defend and relieve the Philippine islands group and the costly attempt to retake them. The senior US Army planner, Brigadier General Stanley D. Embick, stated the case as follows: If the USA adopted as its peace-time frontier in the Pacific the line linking Alaska and Panama via the Hawaiian islands group, the USA’s vital interests would be invulnerable and, in the event of war with Japan, the USA would be free to conduct its military (including naval) operations in a manner that would promise success rather than national disaster.

The US Navy’s planners found this point of view to be wholly unacceptable, for the entire structure of the US Navy’s peacetime planning rested on the proposition that the fleet had to be ready for offensive operations in the Pacific in the event that war broke out. Thus to the naval planners it was out of the question to agree to forego the planning of offensive operations to the west of the Hawaiian islands group. For the following two years the US Army and US Navy planners were then engaged in a sporadic dispute about the basic military policy on which they should base plans for fighting a war with Japan. The US Army chief-of-staff, General Malin Craig, clearly agreed with his planners, but was either unable or unwilling to arrange for the two services' disagreement to be placed before the president for ant form of binding decision.

The weakness of the US strategic situation in the Far Fast and the danger of war became steadily more evident from the mid-1930s. The expiry of the naval limitation agreements led one again to the possibility that the USA might fortify Guam, thus partially offsetting the Japanese position in the mandated island groups, which the USA presumed were being fortified as Japan refused all access to them and intelligence provided no information about them. The Congress refused to authorise Guam’s fortification, however. In the summer of 1937 the Japanese began the undeclared 2^nd Sino-Japanese War by means of the stages 'China Incident', and this brought closer the moment at which the USA would have to choose either to accept or to contest Japan’s aims.

The planners finally reached agreement by avoiding the disputed issues, and at a time early in 1938 submitted a revised plan, which the Joint Board, the US Army chief-of-staff, the Chief of Naval Operations, and the secretaries of the US Army and the US Navy immediately approved. The naval planners agreed to eliminate references to an offensive war, the mission of destroying Japanese forces and the early movement of the fleet into the western Pacific, in return for the agreement of the army planners to eliminate the proviso that any operations to the west of Midway island would require specific authorisation by the president. The revised plan gave no indication of how long it should take the US Navy to advance into the western basic of the Pacific and tacitly recognised the impossible situation of the US forces in the Philippine islands group. These forces retained the basic mission of holding the entrance to Manila bay in order to deny this bay to Japanese naval forces, but no real hope of reinforcement was offered to them.

The increasing threat of war with Japan was in keeping with the growing insecurity of all international relations during the 1930s, and every nation with which the USA had extensive political and economic relations was affected by the prolonged economic crisis of the 1930s and also by its social and political consequences. In Europe the principal factors to be taken into consideration were the revival of German military power and objectives brought into the open by the Nazi government, and the essential passivity of the British and French governments, each of which was paralysed by a dire combination of domestic political conflict, a basic dread of a renewal of war of the type which had cost their nations so dearly in World War I, and acute financial difficulties.

In 1938 the US military staff extended the scope of its contingency planning to take account of the reassertion of German territorial imperialism. The immediate cause was the German demand made on Czechoslovakia in September 1938 for the cession of a strip of territory along its border with Germany. This Sudetenland area contained a large German-speaking minority, among whom the Nazi leadership had recently fomented an irredentist movement in order to create a pretext for German intervention. The area also contained strong border defences and a highly developed munitions industry, which made it by far the most important area, for purely military purposes, in central Europe.

The German ultimatum, backed by German troops mobilised on the border with Czechoslovakia, amounted to a demand that Germany be recognised and accepted as the dominant military power on the European continent, confirming what had in fact been an objective of German domestic and foreign policy since accession to power of Adolf Hitler in 1933. After consolidating his power at home, Hitler had accelerated German rearmament, reintroduced military conscription and remilitarised the Rhineland in direct contravention of the terms of the Treaty of Versailles which had been imposed on Germany in 1919 to bring World War I to a formal end. After his rise to power, Hitler had greatly strengthened the German position and weakened the British and French position in central Europe and the Mediterranean by establishing an alliance with Italy, in which Benito Mussolini’s regime was already embarked on a comparable programme of one-party tyranny, chauvinism and military adventurism, by intervening in the Spanish Civil War (1936/39) on the side of the Nationalist insurgents and by absorbing Austria. To complement these military measures Hitler had cleverly and to a large extent successfully neutralised foreign opposition by the subsidisation of parallel political movements, propaganda and treason, and also by negotiating bilateral trade arrangements and cartel agreements.

Assessing the value of the French alliance with Czechoslovakia and the USSR against their own political and military unpreparedness, the British and French governments were faced by an extremely hard decision. After conferences at Berchtesgaden and Munich, Neville Chamberlain and Edouard Daladier, the British and French prime ministers, agreed not to oppose the German ultimatum and, in so doing, removed from Germany the spectre of having once again to fight on two fronts at the same time, as had been the case in World War I. Moreover, in abandoning Czechoslovakia, which became essentially indefensible after the loss of the Sudeten area, the UK and France significantly weakened the military alliance between France and the USSR. The British and French pusillanimous acquiescence constituted a clear admission of the two countries' political and military weaknesses, and indeed exacerbated them.

After the Munich agreement of September 1938, the prospect of a European general war, which had briefly seemed imminent, seemed to recede, but the military situation in Europe was in fact considerably more threatening than it had been before this. Roosevelt warned the US people that the danger had a bearing on the security of the USA and warned the world at large that the USA fully recognised this danger and would act to meet it, specifically in the western hemisphere. Roosevelt’s declaration carried very little weight either domestically or internationally, however: the media reports and the warnings which accompanied them had no real effect except perhaps to confirm the widespread US belief, shared and expounded by many well-known figures, that the USA should not heed or accept the risk of being drawn into another European war. Roosevelt could neither change nor ignore that expression of US isolationism, and this fact was as well known by his military subordinates as by his political adherents and opponents and the heads of foreign governments. Even so, Roosevelt’s evident concern served to convince the military planners that they should undertake a study, within narrow limits, of the possible effects on US security of action by German action with the support of Italy and, perhaps, of Spain, and action Japan; the planners also started to take into consideration the possibility of joint action by Germany and Japan, possibly with Italian collaboration.

Early in November 1938 the Joint Board told the Joint Planning Committee to study and assess the various practicable courses of action open to the US military and naval forces in the event of a violation of the Monroe Doctrine (US foreign policy dictum regarding Latin American countries established in 1823 to the effect that efforts by European nations to colonise land or interfere with states in North or South America would be viewed as acts of aggression and eliciting US intervention) by one or more of the Fascist powers, and a simultaneous attempt by the Japanese to increase their influence in the Philippine islands group.

The planners undertook a major examination of the problem during the winter of 1938/39, and presented the result in April 1939. Their report listed the advantages Germany and Italy would stand to gain by a violation of the Monroe Doctrine and described the form such a violation might take. What Germany and Italy would try to achieve would be the establishment of 'German and Italian regimes that would approach or attain the status of colonies', with the usually alleged attendant advantages such as increased trade, access to raw materials, and military and naval bases. They might acquire bases 'from which the Panama Canal could be threatened to an extent that pressure could be exerted on United States Foreign Policies'. The probable means of German and Italian aggression to achieve these objectives would be 'direct support of a fascist revolution'. The planners concluded that the danger of this kind of offensive action in the western hemisphere would come to pass only in the event that Germany was confident that the UK and France would not intervene, and in the event that Japan had already attacked the Philippine islands group or Guam in the Mariana islands Group, and even then only in the event that the USA had responded to the Japanese aggression by a counter-offensive into the western basin of the Pacific.

The planners considered it relatively unlikely that in the near future the UK and France would give Germany the necessary assurances or that Japan would decide to attack, but nonetheless believed that the kind of problem posed, resulting from concerted aggression by Germany, Italy and Japan, was one that should be taken into account in future planning, and recommended that steps be taken 'to overcome salient deficiencies in our readiness to undertake the operations that might be required'.

After this study had been approved by the Joint Board, the planners' next effort was to determine the primary courses of action open to the USA as a belligerent in the crises that seemed most likely to develop from future German and Japanese moves, and the delayed responses to them in US foreign and domestic policy. They proposed to assume that initially 'the Democratic Powers of Europe as well as the Latin American States' would be neutral, also proposed to set forth in each situation that might eventuate 'the specific cooperation that should be sought' from these powers at best as allies or at worst as neutrals and, moreover, to provide for possible action in the event that the USA 'should support or be supported by one or more of the Democratic Powers', that is the UK or France.

This projected series of new plans was to be designated as the 'Rainbow War Plans' to distinguish them from the 'Color-coded War Plans' developed in the 1920s and upgraded in the 1930s for operations against one or another single power.

The most limited plan was 'Rainbow 1' (cancelled in May 1942), which was to provide for the defence of the western hemisphere as far to the south as the eastward bulge of Brazil (10° S), the western hemisphere being construed as including Greenland but not Iceland, the Azores islands Group and the Cape Verde islands group to the east, and US Samoa, the Hawaiian islands group and Wake island but not Guam or the Philippine islands group, to the west. Two other plans would provide alternatively for the extension of operations from this area either to the western basin of the Pacific ('Rainbow 2', cancelled in August 1941) or to the rest of South America ('Rainbow 3', cancelled in August 1941). The directive also called for modification of the first three plans under the 'Rainbow 4' (cancelled in May 1942) contingency that the UK and France were at war with Germany and Italy, and possibly with Japan, in which case it was assumed that the USA would be involved as a major participant.

After working for a few weeks under these terms of reference, the Joint Planning Committee concluded that the requirements under this fourth contingency were 'different and divergent' from those in the three basic plans, and that separate plans would have to be made to deal with them. The planners made it clear that in case of war among the great powers, using currently available forces, with the UK, France and possibly the USSR opposing Germany, Italy, Japan and possibly Spain, German and Italian operations in the western part of the Atlantic and in South America would be very much restricted in scope, whereas Japanese operations in the Pacific might be very much extended in scope. If they were not opposed, the Japanese might take the opportunity to take 'the English and French Islands in the South Pacific, east of 180^th meridian, such as Marquesas, Societies, Samoa, and Phoenix Islands, as well as the extensive English and French possessions in the Western Pacific, and the United States possessions in the Pacific'.

Thus the committee suggested that in addition to the three plans against the contingency of a war with Germany, Italy and Japan, two plans should be drawn up to cover the eventuality that a war in which not only the USA but also the UK and France were involved against that coalition: one of the suggested new plans should provide for a large-scale US commitment against Germany, and the other a major US commitment against Japan. The committee stated these two cases as being the USA, UK and France opposed to Germany, Italy and Japan, with the USA providing maximum participation, in particular as regards armies in Europe; and the USA, UK and France opposed to Germany, Italy and Japan With the USA providing maximum participation in the European continent but maintaining the Monroe Doctrine and carrying out allied tasks in the Pacific.

The committee believed the latter of these, which US Navy staff officers had independently been discussing with the British naval staff in ever more definite terms since 1934, to be especially important as it involved problems 'that might conceivably press more for answers' than all but the first and most limited 'Rainbow 1' basic plan. The committee therefore recommended that it should be placed second in order of priority in the list of five situations to be studied, explaining 'Whether or not we have any possible intention of undertaking a war in this situation, nevertheless we may take measures short of war, and in doing so should clarify the possible or probable war task that would be involved.'

On 30 June 1939 the Joint Board approved the recommended changes, including the recommended change in the order of priorities. The revised description of the 'Rainbow War Plans' thus came to be as follows.

'a. Joint Army and Navy Basic War Plan Rainbow No. 1:
Prevent the violation of the letter or spirit of the Monroe Doctrine by protecting that territory of the Western Hemisphere from which the vital interests of the United States can be threatened, while protecting the United States, its possessions and its sea-borne trade. This territory is assumed to be any part of the Western Hemisphere north of the approximate latitude ten degrees south.

'This plan will not provide for projecting U. S. Army Forces farther south than the approximate latitude ten degrees south or outside of the Western Hemisphere.

'b. Joint Army and Navy Basic War Plan Rainbow No. 2:
(1) Provide for the missions in a.
(2) Under the assumption that the United States, Great Britain, and France arc acting in concert, on terms wherein the United States does not provide maximum participation in continental Europe, but undertakes, as its major share in the concerted effort, to sustain the interests of Democratic Powers in the Pacific, to provide for the tasks essential to sustain these interests, and to defeat enemy forces in the Pacific.

'c. Joint Army and Navy Basic War Plan Rainbow No. 3:
(1) Carry out the missions of the Joint Army and Navy Basic War Plan – Rainbow No. 1.
(2) Protect United States' vital interests in the Western Pacific by securing control in the Western Pacific, as rapidly as possible consistent with carrying out the rapidly in a.

'd. Joint Army and Navy Basic War Plan Rainbow No.4:
(1) Prevent the violation of the letter or spirit of the Monroe Doctrine by protecting all the territory and Governments of the Western Hemisphere against external aggression while protecting the United States, its possessions, and its sea-borne trade. This Plan will provide for projecting such U. S. Army Forces as necessary to the southern part of the South American continent or to the Eastern Atlantic.

'e. Joint Army and Navy Basic War Plan Rainbow No. 5:
(1) Provide for the missions in a.
(2) Project the armed forces of the United States to the Eastern Atlantic and to either or both of the African or European Continents, as rapidly as possible consistent with carrying out the missions in a above, in order to effect the decisive defeat of Germany, or Italy, or both. This plan will assume concerted action between the United States, Great Britain, and France.'

This analysis of possible courses of action was easily adaptable to the situation which came to exist for several months after the outbreak of war in Europe in September 1939. When the Germans invaded Poland in 'Weiss' (i), the planning staffs were already fully engaged in the development of plans for a war in the Pacific against Japan, in which the USA would be allied with the European colonial powers within the terms of reference of 'Rainbow 2'. Work on 'Rainbow 2' continued during the autumn of 1939, winter of 1939/40 and spring of 1940. This period witnessed the German and Soviet conquest and dismemberment of Poland, the Soviet 'Winter War' against Finland, and the Sitzkrieg' ('Phoney War') inaction on the Western Front, and as expected 'Rainbow 2' seemed to be the plan most appropriate to the current military situation. The UK and France were at war with Germany, they controlled north-western Europe and northern Africa, and their navies controlled the Atlantic and, to a lesser degree, the North Sea and the Mediterranean. It could thus be assumed that only a Japanese attack would draw the USA into war, and that in the event of any Japanese attack the USA would take precautions in the western hemisphere, and embark, with the approval of the British and French governments, on a process 'to sustain the interests of Democratic Powers in the Pacific, to provide for the tasks essential to sustain these interests, and to defeat enemy forces in the Pacific'.

As the Joint Planning Committee had foreseen, planning against this contingency was complex. The planners had to take into consideration a war considerably more complex than that which they had envisaged in the 'Orange War Plan', with an immensely greater range of possible Japanese operations to consider, and with the very tricky problem of how best to integrate US operations with those of the forces of Australia, New Zealand and the European powers concerned.

The planners first had to assume how far to the south and west the Japanese would have extended their control at the moment the USA and the other 'Democratic Powers' began to react. From the start, the US Navy’s planners established three alternative hypotheses. The first was that Japan would not have begun moving to the south of Formosa, which had been in its possession since the defeat of China in the 1^st Sino-Japanese War (1894/95). In that eventuality major US Navy forces might move to Manila Bay, 'with certain groups visiting Singapore, Kamranh Bay [sic], and Hong Kong'. Ground forces might be moved to the western basin of the Pacific at the same time or later. The US Navy planners thought that these acts might prevent the Japanese from moving to the south, and hence prevent a war in the Pacific. The second hypothesis was that Japan had taken Hong Kong and Cam Ranh Bay, and begun operations in the Netherlands East Indies, in which eventuality the USA would react by moving forces to the far Pacific, and that the Japanese in turn would begin operations to seize Guam and the Philippine islands group. The third hypothesis was that the Japanese would already have control of the Netherlands Indies and would have forces in position to isolate Singapore and take the Philippine islands group, in which eventuality, as the US Army planners pointed out, 'the principal advantages of Allied participation will have been lost and the problem becomes essentially that of an Orange War.'

As major operations in the South-West Pacific seemed less likely under the first and third hypotheses, the planning for 'Rainbow 2' went ahead on the basis of the second hypothesis, which was based on the concept that the Japanese had taken Hong Kong, occupied Cam Ranh Bay in French Indo-China, dominated the coast of French Indo-China and initiated operations against the Netherlands East Indies (in fact including British Borneo), and that Japan has forces available to undertake immediate operations against Guam and the Philippine islands group as soon as it became clear that US armed forces would be deployed in strength into the western basin of the Pacific.

In this case, the most important initial movement of US forces in the Pacific would be to Singapore and the Netherlands East Indies. The US Army planners emphasised that the process of retaking the areas occupied by the Japanese would be slow and necessarily undertaken on a step-by-step basis, and that 'even a day’s delay' in the arrival of US forces would provide the Japanese the opportunity to establish defences whose defeat would require months. As a result, the US Army planners added, it might be necessary to delay operations against the mandated islands and to take into account the danger that the Japanese might cut the US and allied lines of communication through the South Pacific unless the extension of the Japanese forces' own lines of communication might have forced them to effect a significant weakening of their forces in the mandated islands. To avoid this danger, US forces would move to Singapore not via the Philippine islands group but rather by way of the South Pacific: Canton island, Phoenix island, Suva in the Fiji islands group, Rabaul on New Britain island, the Molucca Sea and the Java Sea. These forces would be supplied over the long route across the Atlantic, round the Cape of Good Hope and across the Indian Ocean, although the planners expected that the USA had the capability to despatch air reinforcements by way of the South Pacific, either along the 'island-hopping' route noted above, or by a more southern route from Hawaii to Palmyra, Christmas, Canton and Hull islands, Suva, New Caledonia, New Guinea, Port Darwin in northern Australia, and Soerabaja on Java.

In such a war the joint tasks, in alliance with British, French and Dutch forces, would be to establish U S forces in the Netherlands East Indies, obtain control of the area and drive the Japanese out before the USA could dictate the terms of a peace settlement ensuring that the Japanese evacuated their forces from Hong Kong, the Philippine islands group and Guam.

In seeking assumptions about Japan’s military position at the time that the USA started to react, the planners also encountered a second problem, namely a total uncertainty about the course of action which the European colonial powers might adopt. By April 1940, therefore, the planners had gone as far as they could without having an explicitly approved basis for assuming what the European colonial powers would do. Though not prerequisite of planning for joint action by the US and British navies, which was already well advanced on the basis of Roosevelt’s implicit approval, this was essential even to a hypothetical exploration of the possibility of despatching US Army forces to defend European colonial possessions in the Far East, which was a matter which would have major ramifications for US domestic politics. In this circumstance, therefore, the planners had no option but to recommend that the US government should propose conversations with the British, French and Dutch governments 'as soon as the diplomatic situation permits'. The planners also recommended that the diplomatic conversations 'should be conducted in coordination with representatives of the Chief of Staff of the Army and the Chief of Naval Operations'.

It was logical for the planners to expect that the role of the USA in coalition strategy would be to protect and, if necessary, defend and re-establish its own position and that of the European powers in the western basin of the Pacific. The planners had selected this hypothesis after taking into account the physical facts of the military situation at the beginning of World War II. So far as it went, their analysis of the US role was essentially correct, and later played a major part in strategic planning throughout World War II as the assumptions and plans for 'Rainbow 5' were discussed extensively in the 'Plan Dog' memorandum, which concluded ultimately that the USA would adhere to a 'Europe first' strategy in World War II.

Thus 'Rainbow 5', which was completed in October 1941, was based on the premise of a US alliance with France and the UK, and provided for offensive operations both in the Pacific and in Europe or Africa, or indeed Europe and Africa. The 'Rainbow 5' plan in fact became the basis of American strategy from December 1941, and in its more developed projections for Pacific operations mooted a US Navy-led advance through the Marshall, Caroline and Marianas island groups to produce a decisive naval encounter near Japan’s home waters. The implementation of this basic concept by Admiral Chester W. Nimitz’s Pacific Ocean Areas command was agreed in ABC-1 (spring/summer 1942), but later modified to take account of the nature and strength of General Douglas MacArthur’s South-West Pacific Area command.