What led to World War II and what changed in Europe's map between WWI and WWII?

This is a spectacularly complicated and debated question. I’ll attempt to give you an overview of prominent causes, but be aware that this is a topic historians dedicate their lives to, so there will always be more perspectives and more nuance.

Part 1: The Great War

Fully understanding WWII requires at least an overview of the preceding World War, especially its concluding peace treaty, the Treaty of Versailles, handed by the allied governments to Germany in 1919. World War I was caused by a number of factors, among them the “German Problem,” the issue that a united Germany was “too big” for Europe — it could neither dominate the continent nor be dominated, which created a geopolitical dilemma for the other European powers. At the height of the Empire, Germany controlled the core German lands of what is today Germany, a territory known as East Prussia, which comprised the former lands of the Kingdom of Prussia on the Baltic coast of what is now Poland and Russia, Schleswig-Holstein, the southern part of the Jutland peninsula shared with Denmark, Alsace-Lorraine, a French province conquered in the Franco-Prussian War, and numerous colonial possessions. At Versailles, the victorious powers carved up these territories and distributed them to new states (Poland, which emerged from the treaty with a corridor to the sea carved from East Prussia and port access at Danzig), their former owners (in the case of Alsace-Lorraine, which was returned to France), and the allied powers themselves (for example, Japan gained some German colonial Pacific this possessions). Critically, the Versailles treaty places the blame for the war on Germany — despite the precipitating cause being a crisis in the Balkans between Austria-Hungary and Serbia — and required massive economic concessions in the form of reparations and the cession of the Saar coalfields to France. The treaty was, of course, very unpopular in Germany. It also failed to deal with the Germany Problem — the response in World War II was to divide Germany, which would last until 1989.

Part 2: The Weimar Government and the Rise of Hitler

Inter-War Germany before Hitler seized power is known as the Weimar Republic, so called because it’s first legislature met in the city of Weimar rather than the capital Berlin, which was occupied by protesters at the time. The Weimar Government was rather weak, and inflation rapidly became a problem. German currency rapidly became worthless. This exacerbated underlying tensions about the Treaty of Versailles, which many blamed for Germany’s economic hardship. The effects of the Great Depression did nothing but compound the economic tailspin in Germany.

Adolf Hitler, a veteran of the First World War, became involved with the NASDP, the National Socialist, or Nazi, party in the 1920s. After staging a coup within the party and taking control, Hitler built on strong nationalist themes and began spreading his ideology, which was anti-Communist, anti-Jewish, and racist, among other things. Hitler managed to win a slim majority in the parliament after a fire was blamed on the Communist party, and was empowered by a number of decrees with a huge amount of political power. He rapidly began taking steps to overturn the Treaty of Versailles and restore Germany to its former state.

Part 3: Diplomacy

A number of diplomatic incidents were the direct prelude to the outbreak of World War II, mostly related to Hitler’s foreign policy. In 1934, Hitler attempted to gain control over Austria in what was known as the July Putsch, but a united response from Britain, France, and Fascist Italy and elements within Austria prevented the Nazis seizing control. In 1935, Hitler continued a rearmament campaign in violation of the Treaty of Versailles, and rolled into the Rhineland — demilitarized under the Treaty of Versailles, a 50-mile buffer zone on the east bank of the Rhine — actions which went unopposed by France or Britain. By 1938, Hitler was again ready to try his luck, and succeeded in incorporating Austria into the Reich in what is known as the Anschluss. Later that year, Hitler began to foment rebellion among the Sudeten Germans, ethnic Germans residing in the border areas of Czechoslovakia, an area which happened to be heavily fortified and was Czechoslovakia’s main line of defense against German aggression. In response, the British Prime Minister, Neville Chamberlain, met with Hitler in Munich in September, where they reached an agreement that Czechoslovakia would ceed the Sudetenland to Germany and that Germany would refrain from further aggressive action.

This policy of limited concession in exchange for peace is known as appeasement. Hitler was emboldened by repeated concessions, and took no time invading Czechoslovakia later that year. However, Hitler had his sights set on Poland next. While France and Britain were adamantly defending Polish sovereignty, Hitler concluded a secret treaty with the USSR, the Molotov-Ribbentrop Pact, to divide up Eastern Europe between the two states and to prevent conflict. This non-aggression Pact would last until Hitler’s invasion of the USSR in 1941. Hitler thus invaded Poland in 1939, and in response the United Kingdom and France declared war on Germany.

Conclusion

As demonstrated by this brief overview, the causation behind World War II is incredibly complex and begins well before Hitler’s rise to power in the 1920s. Key factors were the harsh peace imposed on Germany by the victorious powers in World War I, the disastrous economic conditions of the 1920s-1930s in Germany and the weakness of the Weimar Government, and the line of diplomatic incidents that emboldened Hitler’s foreign policy. This overview does not touch on a number of other important factors, including domestic tensions within Germany, the League of Nations and American foreign policy, the rise of fascism in other European states (Spain and Italy), or the impact of the Treaty of Versailles and World War I on other states like France, the United Kingdom, and Japan. It should, however, provide you with a good starting point to begin further research on the topic.