due to Napoleons lust for a french empire, his conquest led to the unification of the German states and the formation of multi nation alliances, those alliances dragging many nations into world war one, and the treaty of Versailles, which placed blame on the Germans for the war and also huge war reparations leading to a hit on German pride and also hyperinflation of the German Mark, leaving the citizens weak to strong nationalism with historical anti-Semitism (predominantly caused my Martin Luther) allowing Adolf Hitler to easily enter office and take over the government leading to the rise of the third Reich and a reclamation of all German land taken from them by the allied powers of WWI. with weak push back from the allies (Munich agreement) Germany quickly annexed more and more land creating more nationalism and lust for a great German state
Stalin signed a nonaggression pact with Germany because he knew that Europe was on the brink of another major war, and he wanted time to be able to build up the Soviet military.
