The 'Phoney War', which the French called the 'Drôle de guerre' and the Germans the 'Sitzkrieg', was the eight-month period at the start of World War II during which there was only one limited military operation on the Western Front, when French troops launched 'Saar' (3 September 1939/10 May 1940).

Germany began its 'Weiss' (i) invasion of Poland on 1 September, and the 'Phoney War' period began with the Britsh and French declarations of war on Germany during 3 September 1939, though little actual warfare occurred, and ended with the German 'Gelb' invasion of the Low Countries and north-eastern France on 10 May 1940. There was no large-scale military action by France and the UK, which did begin some economic warfare, especially with the naval blockade, and shut down German surface raiders. The two Allied nations drafted elaborate plans for many large-scale operations designed to cripple the German war effort, these plans including the opening a Franco-British front in the Balkans, the invasion of Norway to seize control of Germany’s main source of iron ore, and an embargo against the USSR that was Germany’s main source of oil. By April 1940, the lone execution of the Norway plan was considered inadequate to stop the German offensive.

The quiet of the 'Phoney War' was marked by a small number of Allied actions. In the 'Saar' offensive of September, the French attacked Germany with the intention of assisting Poland, but the offensive achieved very little and after only a few days the French withdrew. In November, the USSR attacked Finland in the 'Winter War', resulting in much debate in France and the UK about an offensive to help Finland, but the forces finally assembled for this campaign were delayed until the 'Winter War' ended in March 1940. The Allied discussions about a Scandinavian campaign caused concern in Germany and resulted in the German 'Weserübung' invasion of Denmark and Norway in April, and the Allied troops previously assembled for Finland were redirected to Norway, where the fighting continued until June, when the Allies forces were evacuated, ceding Norway to Germany in response to the German invasion of France.

On the Axis side, the Germans launched attacks at sea in the autumn and winter against British aircraft carriers and destroyers, sinking several including the carrier Courageous with the loss of 519 lives. Action in the air began on 16 October 1939 when the Luftwaffe launched air raids on British warships, and each side made various minor bombing raids and reconnaissance flights.

The initial term used by British people for this period was Bore War. While this was probably coined as a play on the Boer War, fought some four decades earlier, eventually the Americanism 'Phoney War' found favour on each side of the Atlantic Ocean, probably (and particularly so in the British commonwealth and empire) mainly to avoid confusion with the earlier conflict. The term 'Phoney War' customarily appears with the British spelling even in North America, rather than the US 'Phony', although some US sources do not follow the pattern. The first known recorded use of the term in print was in September 1939, in a US newspaper which used the British spelling, although other contemporary US reports sometimes used 'phony' as both spellings were currently in US use. The term had appeared in the UK by January 1940 as 'phoney'.

The 'Phoney War' was also known as the 'Twilight War' and as the 'Sitzkrieg' (sitting war), the latter a word play on Blitzkrieg created by the British press. In French, the 'Phoney War' became known as the 'Drôle de Guerre' (funny war).

In March 1939, the UK and France formalised existing plans for how a war against Germany would be conducted. Knowing that likely opponents would be better prepared, and possess both land and air superiority, the strategy was to defeat any German offensive and thereby buy the time needed for economic and naval superiority to build up military resources. To this end, the UK initially committed to the despatch of an initial two divisions to France, with two more to follow 11 months later. However, the Polish army’s general plan for defence, 'Zavod', assumed that the Allies' offensive on the Western front would provide significant relief to the Polish front in the east.

While most of the German army was engaged in Poland, a much smaller German force manned the 'Siegfried-Linie', the German fortified defensive line along the French border. On 7 September, the French launched their 'Saar' offensive, but had to withdraw when their artillery could not penetrate the German defences. A further assault was planned for 20 September, but on 17 September, following the USSR’s invasion of eastern Poland, the assault was called off. In the air, the Royal Air Force launched a daylight bombing attack on Wilhelmshaven on 4 September, although this proved costly. There were occasional dogfights between fighters. The RAF dropped propaganda leaflets on Germany, the first Canadian troops arrived in the UK, and the first divisions of the British Expeditionary Force completed their movement to France, while western Europe was under a period of uneasy calm for seven months.

In the first few months of the war, Germany still hoped to persuade the UK to agree to peace. Although London hospitals prepared for 300,000 casualties in the first week, Germany unexpectedly did not immediately attack British cities from the air, and German pilots who attacked Scottish naval bases said that they would have been court-martialled and executed for bombing civilians. Both sides found that attacks on military targets, such as a British attack on Kiel on the second night of the war, led to high losses of aircraft. They also feared retaliation for bombing civilians. British civilian attitudes toward the Germans were still not as intense as they were to become after 'The Blitz'. On 30 April 1940, a Heinkel 111 twin-engined medium bomber crashed at Clacton-on-Sea in Essex, killing its crew and injuring 160 people on the ground, but the crew was buried in the local cemetery with support from the RAF and messages of sympathy displayed on the coffins.

When the member of parliament Leopold Amery suggested to Kingsley Wood, the Secretary of State for Air, that the Black Forest be bombed with incendiaries to burn any ammunition dumps in it, he was amazed when Wood replied that the forest was private property and could therefore not be bombed, and neither could weapons factories as the Germans might do the same.

In their hurry to rearm at the start of the war, the UK and France each bought large quantities of weapons from manufacturers in the USA to supplement their own production. A non-belligerent, the USA contributed to the Western Allies with discounted sales.

Despite the relative calm on land, at sea the war was very real. Within a few hours of the declaration of war, the British liner Athenia was torpedoed off the Hebrides islands group with the loss of 112 lives. On 4 September, the Allies announced a blockade to prevent German imports of food and raw materials to sustain her war effort, the Germans immediately declared a counter-blockade and the USSR helped Germany with supplies bypassing the blockade. RAF Bomber Command, Britain’s principal offensive arm, was also heavily engaged, but found that daylight bombing caused little damage and cost insupportable losses, such as that which took place on 18 December, when 12 out of 22 Vickers Wellington twin-engined medium bombers were shot down in an air battle over the Wilhelmshaven naval base.

At the post-war Nuremberg trials, Generaloberst Alfred Jodl, the chief-of-staff of the operations branch of the Oberkommando der Wehrmacht, said that 'if we did not collapse already in the year 1939 that was due only to the fact that during the Polish campaign, the approximately 110 French and British divisions in the West were held completely inactive against the 23 German divisions', and another senior German commander, General Siegfried Westphal, stated that if the French had attacked in force in September 1939 the German army 'could only have held out for one or two weeks'.

The 'Saar' offensive was a French attack into the Saarland, which was defended by the German 1^st Army, with the object of aiding Poland. The assault was ordered to halt after an advance of only a few kilometres and the French forces then withdrew. According to the Franco-Polish military convention, the French army was to have started preparations for a major offensive three days after the beginning of mobilisation, and was to gain effective control over the area between the French border and the German lines and to probe the German defences. On the fifteenth day of the mobilisation (16 September), the French army was to have begun a full-scale assault on Germany. France started a pre-emptive mobilisation on 26 August, and on 1 September ordered full mobilisation. The offensive in the Rhine river valley area started on 7 September, four days after France declared war on Germany. Since the German forces were occupied in the attack on Poland, the French had a decisive numerical advantage along their border with Germany. A total of 11 French divisions advanced along a 20-mile (32-km) front near Saarbrücken against weak German opposition, but the attack did not result in the diversion of any German troops. The all-out assault was to have been carried out by roughly 40 divisions, including one armoured and three mechanised divisions, 78 artillery regiments and 40 tank battalions. The French had advanced to a depth of 5 miles (8 km) and captured about 20 villages evacuated by the German army without meeting any resistance, and this half-hearted offensive was halted after French forces had seized the Warndt forest, 3 sq miles (7.8 km²) of heavily mined German territory.

On 12 September, the Anglo-French Supreme War Council gathered for the first time at Abbeville. It was decided that all offensive actions were to be halted immediately as the French preferred to fight a defensive war, forcing the Germans to come to them. Général d’Armée Maurice Gamelin, the French commander-in-chief, ordered his troops to come to a halt no closer than 0.6 mile (1 km) from the German positions along the 'Siegfried-Linie'. Poland was not notified of this decision, but instead Gamelin informed Marszałek Polski Edward Rydz-Śmigły, the Poilish commander-in-chief, that half of his divisions were in contact with the enemy and that French advances had forced the Germans to withdraw at least six divisions from Poland. On the following day, the commander of the French military mission to Poland, Général de Division Louis Faury, informed the Polish chief-of-staff, General dywizji Wacław Stachiewicz, that the major offensive on the western front planned from 17/20 September had been postponed. At the same time, French divisions were ordered to withdraw to their barracks along the 'Ligne Maginot'.

A notable event during the 'Phoney War' was the 'Winter War', which started with the USSR’s assault on Finland from 30 November 1939. Public opinion, particularly in France and the UK, found it easy to side with Finland and demanded from their governments effective action in support of 'the brave Finns' against their much larger aggressor, all the more so as the Finns' defence seemed so much more successful than that of the Poles during 'Weiss' (i). As a consequence of its attack, the USSR was expelled from the League of Nations, and the proposed despatch of a Franco-British expedition to northern Scandinavia was much debated. The assembly of British forces for despatch to Finland’s aid was not completed, nor were any forces despatched before the 'Winter War' ended, but were sent instead to Norway’s aid in the Norwegian campaign. On 20 March, after the 'Winter War' had ended, Edouard Daladier resigned as the French prime minister, in part as a result of his failure to aid Finland’s defence.

The open discussions on an Allied expedition to northern Scandinavia, also without the consent of the neutral Scandinavian countries, and the 'Altmark Incident' of 16 February, alarmed the Kriegsmarine and Germany by threatening the flow of high-grade iron ore from Sweden via the ice-free Norwegian port of Narvik, and gave strong arguments for Germany to secure the Norwegian coast. Codenamed 'Weserübung', the German invasion of Denmark and Norway began on 9 April. From 14 April Allied troops landed in Norway, but by the end of the month the southern parts of Norway were in German hands. The fighting continued in the north until the Allied forces were evacuated early in June in response to the German invasion of France; the Norwegian forces in mainland Norway laid down their arms at midnight on 9 June.

The debacle of the Allied campaign in Norway forced a debate in the House of Commons during which the British prime minister, Neville Chamberlain, was under constant attack. A nominal vote of confidence in his government was won by 281 votes to 200, but many of Chamberlain’s supporters had voted against him while others had abstained. Chamberlain thus found it impossible to continue to lead a national government or to form a new coalition government with himself as leader, and on 10 May Chamberlain resigned the premiership but retained the leadership of the Conservative Party. Winston Churchill, who had been a consistent opponent of Chamberlain’s policy of appeasement, became Chamberlain’s successor. Churchill formed a new coalition government that included members of the Conservative, Labour and Liberal parties, as well as several ministers from non-political backgrounds.

British war planning had called for a 'knock-out blow' by strategic bombing of German industry with the RAF’s substantial Bomber Command. However, there was considerable apprehension about German retaliation, and when President Franklin D. Roosevelt of the USA proposed an agreement not to mount any bombing raids which might endanger civilians, the UK and France agreed immediately and Germany agreed two weeks later. The RAF therefore conducted a large number of combined reconnaissance and propaganda leaflet flights over Germany. These operations were jokingly termed 'pamphlet raids' or 'confetti war' in the British press.

On 10 May 1940, eight months after the British and French declarations of war on Germany, German troops marched into Belgium, the Netherlands, Luxembourg and France, bringing the 'Phoney War' and the beginning of the 'Battle of France'.

Italy, hoping for territorial gains when France was defeated, entered the war on 10 June 1940, although the 32 Italian divisions which crossed the border with France enjoyed little success against five defending French divisions.