Germany emerged from World War One a defeated nation, her army beaten on the field and her allies collapsing under the strain of four years of unrelenting war. In the economic chaos that followed the war, the far-right National Socialist German Workers' Party led by the charismatic Adolf Hitler gained massive popularity.
Following Hindenburg's death, the NSDAP assumed full control of Germany. The NSDAP transformed Germany into a one-party totalitarian regime and persecuted Communists, Socialists, Jews, and other "undesirables". In 1936, Germany made it's first move towards expansion when it re-militarized the Rhineland. The French and British failed to stop them. In 1938, the Federal State of Austria was peacefully annexed following a pro-Nazi coup.
During the "Sudetenkrise", the NSDAP used pan-German sentiments to spark conflict with the First Czechoslovak Republic. The primary goal was the unification of ethnic Germans living in Czechoslovakia with those living in the German Reich (at the time, ethnic Germans made up 90% of the Sudetenland and 20% of the whole Czechoslovak state). To settle the matter, the Munich Conference was held and Czechoslovakia was forced to give up the Sudetenland to Germany. The next year brought about the annexation of the rest of Czechoslovakia (which the Allies could only protest). It was at this point, Britain and France declared their support of Poland in case of a German invasion.
Germany demanded to annex the historically disputed Danzig Corridor, that separated East Prussia from the German mainland. Upon Poland's refusal, an invasion occurred, in which, both Germany and the USSR invaded Poland. To ensure Polish sovereignty, England and France declared war on Germany, sparking World War II.