The National Socialist German Workers Party (AKA NSDAP, AKA Nazis) rose to power in Germany. Their first move was to remilitarize the Rhineland in 1936 and regain the ability to remobilize their military.
Supposedly Hitler (Again)
Nazi's and Nazi Supporters
After the Germans remilitarized the Rhineland they annexed Austria (Both countries wanted to be unified but were barred from doing so due to the treaty of versailles) then a few years later demanded the Sudetenland from Czechoslovakia which, due to the Munich Conference, they successfully integrated into the German Reich. The Germans were greedy later on though and demanded the rest of Czechoslovakia and took Bohemia and instilled a puppet government in Slovakia (Though Carpatho Ukraine was liberated shortly after and instantly attacked by the Hungarians)
On September 1st, 1939, the second World War began and Europe was thrown into chaos. In the following years more than 50 million people died and the political map of Europe had been changed forever...
Chamberlain was afraid of starting another World War so soon after the first one had ended so he appeased the Germans, allowing them to remilitarize the Rhineland, unify with Austria, and take Czechslovakia.
1939, Germany invades Poland for their refusal to cede Danzig (Or Gdansk if you're polish) Unfortunately for the Germans France and the UK were sick of Germany's constant warmongering, so as soon as the Germans attacked the Polish, France and the UK declared war on them.