What Was The Main Cause Of World War Two?

World War Two began with Britain and France declaring war on Germany as a response to Germany's invasion of Poland in September1939.  But the Germans had long been preparing for war as their own response to the disastrous fallout, for them, from their defeat in World War One.  

Hitler and his Nazi party rose to power on the back of deep resentment felt in Germany since the 1919 Treaty of Versailles, the peace settlement that forced Germany to admit responsibility for World War One, stripped it of some home territories and its overseas colonies, and ordered it to pay huge financial 'reparations' to the winning allies.  It also placed restrictions on the size and equipment of the German army but, from 1933, Germany began to rearm.  Hitler had rejected the Treaty of Versailles and was preparing to re-unite all Germans into one nation.  His plans included taking East European territory as 'space to live' for Germans.  Despite the flouting of the Treaty, Britain opted for appeasement and Germany grew stronger and continued to break the Treaty, with re-entry of the Rhineland in 1936 (no French resistance) and annexing of Austria followed by occupation (sanctioned by Britain and France) of the Sudetenland in Czechoslovakia in 1938.  

Hitler's relentless land grab could not be appeased forever.  Once it became clear that force was the only way to stop Germany's illegal expansion, war became inevitable.