quality:corruption
Start of World War II

In private discussions in 1939, Hitler declared Britain the main enemy to be defeated and that Poland's obliteration was a necessary prelude for that goal. The eastern flank would be secured and land would be added to Germany's Lebensraum. Offended by the British "guarantee" on 31 March 1939 of Polish independence, he said, "I shall brew them a devil's drink". In a speech in Wilhelmshaven for the launch of the battleship Tirpitz on 1 April, he threatened to denounce the Anglo-German Naval Agreement if the British continued to guarantee Polish independence, which he perceived as an "encirclement" policy. Poland was to either become a German satellite state or it would be neutralised in order to secure the Reich's eastern flank and prevent a possible British blockade.

Hitler initially favoured the idea of a satellite state, but upon its rejection by the Polish government, he decided to invade and made this the main foreign policy goal of 1939. On 3 April, Hitler ordered the military to prepare for Fall Weiss ("Case White"), the plan for invading Poland on 25 August. In a Reichstag speech on 28 April, he renounced both the Anglo-German Naval Agreement and the German–Polish Non-Aggression Pact. Historians such as William Carr, Gerhard Weinberg, and Ian Kershaw have argued that one reason for Hitler's rush to war was his fear of an early death. He had repeatedly claimed that he must lead Germany into war before he got too old, as his successors might lack his strength of will. Hitler was concerned that a military attack against Poland could result in a premature war with Britain. Hitler's foreign minister and former Ambassador to London, Joachim von Ribbentrop, assured him that neither Britain nor France would honour their commitments to Poland. Accordingly, on 22 August 1939 Hitler ordered a military mobilisation against Poland.

This plan required tacit Soviet support, and the non-aggression pact (the Molotov–Ribbentrop Pact) between Germany and the Soviet Union, led by Joseph Stalin, included a secret agreement to partition Poland between the two countries. Contrary to Ribbentrop's prediction that Britain would sever Anglo-Polish ties, Britain and Poland signed the Anglo-Polish alliance on 25 August 1939. This, along with news from Italy that Mussolini would not honour the Pact of Steel, prompted Hitler to postpone the attack on Poland from 25 August to 1 September. Hitler unsuccessfully tried to manoeuvre the British into neutrality by offering them a non-aggression guarantee on 25 August; he then instructed Ribbentrop to present a last-minute peace plan with an impossibly short time limit in an effort to blame the imminent war on British and Polish inaction.

On 1 September 1939, Germany invaded western Poland under the pretext of having been denied claims to the Free City of Danzig and the right to extraterritorial roads across the Polish Corridor, which Germany had ceded under the Versailles Treaty. In response, Britain and France declared war on Germany on 3 September, surprising Hitler and prompting him to angrily ask Ribbentrop, "Now what?" France and Britain did not act on their declarations immediately, and on 17 September, Soviet forces invaded eastern Poland.

The fall of Poland was followed by what contemporary journalists dubbed the " Phoney War " or Sitzkrieg ("sitting war"). Hitler instructed the two newly appointed Gauleiters of north-western Poland, Albert Forster of Reichsgau Danzig-West Prussia and Arthur Greiser of Reichsgau Wartheland, to Germanise their areas, with "no questions asked" about how this was accomplished. In Forster's area, ethnic Poles merely had to sign forms stating that they had German blood. In contrast, Greiser agreed with Himmler and carried out an ethnic cleansing campaign towards Poles. Greiser soon complained that Forster was allowing thousands of Poles to be accepted as "racial" Germans and thus endangered German "racial purity". Hitler refrained from getting involved. This inaction has been advanced as an example of the theory of "working towards the Führer", in which Hitler issued vague instructions and expected his subordinates to work out policies on their own.

Another dispute pitched one side represented by Heinrich Himmler and Greiser, who championed ethnic cleansing in Poland, against another represented by Göring and Hans Frank (governor-general of occupied Poland), who called for turning Poland into the "granary" of the Reich. On 12 February 1940, the dispute was initially settled in favour of the Göring–Frank view, which ended the economically disruptive mass expulsions. On 15 May 1940, Himmler issued a memo entitled "Some Thoughts on the Treatment of Alien Population in the East", calling for the expulsion of the entire Jewish population of Europe into Africa and the reduction of the Polish population to a "leaderless class of labourers". Hitler called Himmler's memo "good and correct", and, ignoring Göring and Frank, implemented the Himmler–Greiser policy in Poland.

On 9 April, German forces invaded Denmark and Norway. On the same day Hitler proclaimed the birth of the Greater Germanic Reich, his vision of a united empire of Germanic nations of Europe in which the Dutch, Flemish, and Scandinavians were joined into a "racially pure" polity under German leadership. In May 1940, Germany attacked France, and conquered Luxembourg, the Netherlands, and Belgium. These victories prompted Mussolini to have Italy join forces with Hitler on 10 June. France and Germany signed an armistice on 22 June. Kershaw notes that Hitler's popularity within Germany—and German support for the war—reached its peak when he returned to Berlin on 6 July from his tour of Paris. Following the unexpected swift victory, Hitler promoted twelve generals to the rank of field marshal during the 1940 Field Marshal Ceremony.

Britain, whose troops were forced to evacuate France by sea from Dunkirk, continued to fight alongside other British dominions in the Battle of the Atlantic. Hitler made peace overtures to the new British leader, Winston Churchill, and upon their rejection he ordered a series of aerial attacks on Royal Air Force airbases and radar stations in southeast England. On 7 September the systematic nightly bombing of London began. The German Luftwaffe failed to defeat the Royal Air Force in what became known as the Battle of Britain. By the end of September, Hitler realised that air superiority for the invasion of Britain (in Operation Sea Lion) could not be achieved, and ordered the operation postponed. The nightly air raids on British cities intensified and continued for months, including London, Plymouth, and Coventry.

On 27 September 1940, the Tripartite Pact was signed in Berlin by Saburō Kurusu of Imperial Japan, Hitler, and Italian foreign minister Ciano, and later expanded to include Hungary, Romania, and Bulgaria, thus yielding the Axis powers. Hitler's attempt to integrate the Soviet Union into the anti-British bloc failed after inconclusive talks between Hitler and Molotov in Berlin in November, and he ordered preparations for the invasion of the Soviet Union.

In early 1941, German forces were deployed to North Africa, the Balkans, and the Middle East. In February, German forces arrived in Libya to bolster the Italian presence. In April, Hitler launched the invasion of Yugoslavia, quickly followed by the invasion of Greece. In May, German forces were sent to support Iraqi forces fighting against the British and to invade Crete.