The allies signed the Treaty of Versailles which forced Germany to pay back war reparations to certain countries including Great Britain, France, and the USA.
One of the terms of the Treaty of Versailles was that Germany bad to have a limited army. Hitler, however, violated the treaty and secretly began to build up his army expecting there to be another world war.
The great depression caused global depression. Global depression caused the collapse of Germany's industry which then led directly to Hitler's rise to power also evoked by the rise of inflation and unemployment.
As Hitler started to invade countries and build a military capable of fighting a major war, despite the Treaty of Versailles, Britain and France allowed him to continue, hoping he would leave them alone if they left him alone.
German forces attack Poland on land and from the air using the 'blitzkreig', as Hitler tries to regain lost territory and ultimately take over Poland. The German invasion was a start on how Hitler intended to start WW2.
The United States did not enter the war until after the Japanese bombed the American fleet in Pearl Harbor, Hawaii, on December 7, 1941.