Among the causes of World War II were, to a greater extent, the political takeover in 1933 of Germany by Adolf Hitler and his Nazi Party and its aggressive foreign policy, and to a lesser extent, Italian Fascism in the 1920s, and Japanese militarism preceding an invasion of China in the 1930s. The immediate cause was Germany invading Poland on September 1, 1939, and Britain and France declaring war on Germany on September 3, 1939.
After his rise and take-over of power in 1933 to a large part based on these grievances, Adolf Hitler and the Nazis heavily promoted them and also ideas of vastly ambitious additional demands based on Nazi ideology, such as uniting all Germans (and further all Germanic peoples) in Europe in a single nation; the acquisition of “living space” (Lebensraum) for primarily agrarian settlers (Blut und Boden), creating a “pull towards the East” (Drang nach Osten) where such territories were to be found and colonized; the elimination of Bolshevism; and the hegemony of an “Aryan”/”Nordic” so-called Master Race over the “sub-humans” (Untermenschen) of inferior races, chief among them Slavs and Jews.
Tensions created by those ideologies and the dissatisfactions of those powers with the interwar international order steadily increased. Italy laid claim on Ethiopia and conquered it in 1935, Japan created a puppet state in Manchuria in 1931 and expanded beyond in China from 1937, and Germany systematically flouted the Versailles treaty, reintroducing conscription in 1935 with the Stresa Front’s failure after having secretly started re-armament, remilitarizing the Rhineland in 1936, annexing Austria in March 1938, and the Sudetenland in October 1938.
All those aggressive moves met only feeble and ineffectual policies of appeasement from the League of Nations and the Entente Cordiale – in retrospect symbolized by the “peace for our time” speech following the Munich Conference, that had allowed the annexation of the Sudeten from interwar Czechoslovakia. When the German Führer broke the promise he had made at that conference to respect that country’s future territorial integrity in March 1939 by sending troops into Prague, its capital, breaking off Slovakia as a German client state, and absorbing the rest of it as the “Protectorate of Bohemia and Moravia”, Britain and France tried to switch to a policy of deterrence.
As Nazi attentions turned towards resolving the “Polish Corridor Question” during the summer of 1939, Britain and France committed themselves to an alliance with Poland, threatening Germany with a two-front war. On their side, the Germans assured themselves of the support of the USSR by signing a non-aggression pact with them in August, secretly dividing Eastern Europe into Nazi and Soviet spheres of influence.
The stage was then set for the Danzig crisis to become the immediate trigger of the war in Europe which started on 1 September 1939 when Germany invaded Poland. Britain and France then gave Germany an ultimatum to withdraw, which Germany ignored, and Britain and France declared war on Germany on September 3, 1939.