'Tannenbaum' (i) was a German unrealised operation by the staff of Generaloberst (from 19 July 1940 Generalfeldmarschall) Wilhelm Ritter von Leeb’s Heeresgruppe 'C' for the invasion and occupation of Switzerland (June 1940/June 1944).

For purely operational reasons, Adolf Hitler made repeated assurances before the start of World War II that Germany would respect Swiss neutrality in the event of any outbreak of military conflict in Europe. In February 1937, foe example, he announced that at all times and under all circumstances Germany would the inviolability and neutrality of Switzerland, a statement he reiterated shortly before the 'Weiss' (i) invasion of Poland which triggered World War II in September 1939. These and similar statements were nothing but elements of Germany political manoeuvring designed to ensure the passivity of Switzerland, and Germany planned to end Switzerland’s independence after it had defeated its primary enemies on the European continent.

At a meeting held with Benito Mussolini and Galeazzo Ciano, the Italian leader and foreign minister respectively, in the course of June 1941, Hitler was altogether more open about his feelings with regard to Switzerland, stating that Switzerland possessed the most disgusting and miserable people and political system, and that the Swiss were the mortal enemies of the 'new Germany'. In a later discussion, the German foreign minister, Joachim Ribbentrop, mentioned the possibility of the division of Switzerland between Germany and Italy.

As a small, multi-lingual and federal democracy in which German-speakers felt closer to their French-speaking fellow Swiss rather than toward their 'brothers' across the border in Germany, Switzerland was from the Nazi point of view the antithesis of the racially homogeneous and centralised German state. Hitler also believed that the independent Swiss state had come into existence as a result of the temporary weakness of the Holy Roman Empire and, now that the latter’s power had been re-created by the Nazi assumption of power in Germany, it had become obsolete.

Yet though he despised the democratic German Swiss as a branch of the German people which had gone astray, Hitler nonetheless acknowledged that they were still ethnic Germans who, by the tenets of German aspirations toward the creation of a pan-German state, should be incorporated into the 'new Germany'. The first goal of the 25-point Nazi programme stated that the party demanded the unification of all Germans in the 'Greater Germany' on the basis of the people’s right to self-determination.

In their maps of Greater Germany, German text books included the Netherlands, Belgium, Austria, Bohemia Moravia, the German-speaking parts of Switzerland and the western part of Poland from Danzig (now Gdańsk) to Kraków. Ignoring Switzerland’s status as a sovereign state, these maps frequently showed its territory as a German Gau. The author of one of such text books expounded that 'Quite naturally we count you Swiss as offshoots of the German nation, along with the Dutch, the Flemings, the Lorrainers, the Alsatians, the Austrians and the Bohemians…One day we will group ourselves around a single banner, and whosoever shall wish to separate us, we will exterminate!', and many Nazis were explicit in Germany’s need to expand the German boundaries to the farthest extent, and indeed farther, than the frontiers of the old Holy Roman Empire. One German geopolitician, though not an adherent of the Nazi party, called for the partition of Switzerland between surrounding countries, with Romandy (Welschland) awarded to Vichy France, Ticino to Italy, and central and eastern Switzerland to Germany.

Before Hitler’s 1935 renunciation of the strictures imposed of Germany by the Treaty of Versailles, the Swiss government had approved an increase in defence spending, with a first instalment of 15% of a 100 million-franc multi-year budget allocated to modernisation. After Hitler’s renunciation, this spending was increased to 90 million francs.

In 1933 the K31 had been adopted as the standard Swiss infantry rifle, and this was better than the German Kar 98 in ease of use, accuracy and weight. Switzerland has a unique form of senior military command. In times of peace, there is no officer with a rank higher than Korpskommandant (lieutenant general), but in times of war and emergency, the Bundesversammlung (federal assembly) elects a full general to command the army and air force. On 30 August 1939, Henri Guisan was elected with 204 of 227 votes, and immediately took charge of the situation.

The German invasion of Poland two days later led France and the UK to declare war on Germany, and at the same time Guisan ordered general mobilisation and issued Operationsbefehl Nr. 1 as the first of what was to be a series of evolving defence plans. The first assigned Switzerland’s three existing corps to the east, north, and west, with reserves mustered in the centre and south of the country. Guisan reported to the federal assembly on September 7 that by the moment of the British declaration of war the whole Swiss army had been in its operational positions for 10 minutes. He also ordered his chief-of-staff to increase the upper age limit of service eligibility from 48 to 60 years, the newly eligible men to form rear-echelon Landsturm local-defence units), and ordered the formation of an entirely new corps of 100,000 men.

In several air incidents, the Swiss (using 10 Messerschmitt Bf 109D and 80 Bf 109E fighters bought from Germany as well as some Morane-Saulnier MS.406 French fighters built under licence) shot down 11 German aircraft between 10 May and 17 June 1940. Germany intervened diplomatically on 5 June, following with a second note on 19 June which contained clear threats. On 20 June the Swiss air force was ordered no longer to intercept aircraft violating Swiss airspace, although anti-aircraft units were still authorised to fire on them. Later Hitler unsuccessfully sent saboteurs to destroy airfields.

Germany started planning an invasion of Switzerland on 25 June 1940, the day on which the surrender of France became effective: at this moment the German army in France comprised three army groups with 102 divisions of some 2 million men. Switzerland and Liechtenstein were completely surrounded by German-occupied France, a small part of Vichy France, Germany and Italy, so Guisan issued Operationsbefehl Nr. 10 as a complete overhaul of existing Swiss defence plans. The St Maurice and St Gotthard passes in the south and the Sargans fortress in the north-east anchored the Swiss defences, and the II, III and IV Corps were to fight delaying actions at the border, while all who could do so fell back into the Alpine refuge known as the Réduit National (national redoubt). Switzerland’s main population centres were all located in the lower ground of the country’s north, and would have to be left to the Germans in order for the rest to survive.

Hitler demanded to see plans for the invasion of Switzerland after the armistice with France. An officer of the Oberkommando des Heeres, Hauptmann Otto-Wilhelm Kurt von Menges, drafted a plan for the invasion. In this, von Leeb’s Heeresgruppe 'C' would undertake the invasion with Generalfeldmarschall Wilhelm List’s 12^th Army in the van. von Leeb personally reconnoitred the terrain, studying the most promising invasion routes and seeking to locate the paths of least resistance. von Menges noted in his plan that Swiss resistance was unlikely and that a non-violent Anschluss was the most likely result, starting that in the light of its current current political situation, Switzerland might agree to ultimatum demands without fighting, and thus an armed invasion would be followed by a peaceful occupation.

The German plan continued to undergo revision until October, when the 12^th Army submitted its fourth draft, now called 'Tannenbaum' (i). The original plan called for the commitment of 21 German divisions, but the Oberkommando des Heeres reduced this to 11 divisions. Generaloberst Franz Halder, the Oberkommando des Heeres’s chief-of-staff, had also studied the border areas, and concluded that the Jura frontier provided no favourable base for an attack as Switzerland’s terrain rose, in successive waves of wood-covered terrain, across the axis of an attack, and that the crossing points on the Doubs river and the border were few and the Swiss frontier positions strong. Halder opted instead for an infantry feint in the Jura in order to draw the Swiss army forward so that the Germans could debouch into the Swiss rear areas and cut its lines of communication, as had been the operational method in the Battle of France. With the 11 German divisions and some 15 Italian divisions prepared to enter the country from the south, the Swiss were facing an invasion by somewhere between 300,000 and 500,000 men.

There are several possible reasons for Germany’s early non-intervention in Switzerland. First, Switzerland was not seen as any form of direct threat to Germany. Second, Hitler had his thoughts centred on the Battle of Britain and the planned 'Seelöwe' invasion of the UK, the latter including Germany’s few mountain divisions. Third, Hitler was planning the invasion of the USSR in 'Barbarossa' and already, by August and September 1940, large numbers of men and considerable quantities of weapons and supplies had been moved to the east to counter a possible Soviet threat to Bessarabia.

The main window of opportunity for German military action against Switzerland was the period between the fall of France in June 1940 and November 1940, after which the weather would not have permitted a real Blitzkrieg, and after the winter of 1940/41 Hitler was occupied with the preparations for 'Unternehmen 25' and 'Marita' as well as 'Barbarossa'.

In the longer term, Hitler still did not give the signal for 'Tannenbaum' (i) to proceed, for reasons which are uncertain, but perhaps because Switzerland had defences of a type possessed by no other country: every Swiss home was equipped with a rifle, and the alpine terrain gave a tactical advantage to a Swiss army, totalling one-fifth of the population. The Swiss government was based on a decentralised structure, moreover, so even the federal president was a relatively powerless official with no authority to surrender the country: indeed, Swiss citizens had been instructed to regard any surrender broadcast as enemy lies and resist to the end. The German forces did feign moves toward Switzerland, however. After the start of the Allied 'Overlord' landing in north-western France on 6 June 1944, 'Tannenbaum' (i) was put on hold, and Switzerland continued to remain inviolate for the duration of the war.

The German political objective in the expected conquest of Switzerland was to regain the bulk of the 'racially suitable' Swiss population for the Germanic race, and to annexe into the Germany at least its ethnic German parts.

With this in mind, during September 1941 Reichsführer-SS Heinrich Himmler discussed with SS-Obergruppenführer Gottlob Berger of the SS-Hauptamt (head office) the suitability of a number of officials for the position of the Reichskommissar who would supervise the 'reunion' of Switzerland with Germany and then become its Reichsstatthalter (Reich governor). This official was to facilitate the complete integration of the Swiss into the population of 'Greater Germany'. Himmler further attempted to expand the SS into Switzerland with the formation of the Germanische SS Schweiz in 1942.

A document named Aktion S (the letter denoting Schweiz) was later found within Himmler’s files. This detailed at length the planned process for the establishment of Nazi rule in Switzerland from its initial conquest up to its complete consolidation as a German province. It is not known whether this prepared plan was endorsed by any high-level members of the Nazi government.

After the surrender of France in June 1940, the German interior ministry produced a memorandum on the annexation of a strip of eastern France from the mouth of the Somme river in the north to Lake Geneva in the south as a reserve for post-war German colonisation. The planned dissection of Switzerland would have accorded with this new French-German border, effectively leaving the French-speaking region of Romandy also to be annexed into the Reich despite the linguistic difference.

Germany’s wartime ally Italy, led by Mussolini, desired the annexation of the Italian-speaking areas of Switzerland, and especially the canton of Ticino, as part of its irredentist claims in Europe and, in a tour of the Italian alpine regions told his entourage that the 'New Europe' could be based on no more than four or five large states, and that the smaller ones would lose all reason for their continued existence and woulds have to disappear.

Switzerland’s future in an Axis-dominated Europe was further discussed in a 1940 conference between Ciano and Ribbentrop, also attended by Hitler. Ciano proposed that in the event of its dissolution, Switzerland should be divided along the central chain of the Western Alps, as Italy desired the areas to the south of this demarcation line as part of its own war aims. This would have left Italy in control of Ticino, Valais and Graubünden.