Harsh Treaty of Versailles: The Treaty of Versailles, which was signed in 1919 to end World War I, was very harsh on Germany. It forced Germany to accept responsibility for the war, cede territory to the Allies, pay reparations, and reduce its military strength. This created a sense of resentment and nationalism among Germans, who felt they were being unfairly punished.
Rise of Nazism: The economic instability and political unrest in Germany after World War I led to the rise of Adolf Hitler and the Nazi Party. Hitler promised to restore Germany's greatness and undo the injustices of the Treaty of Versailles. He also promoted racist and anti-Semitic ideas, which appealed to many Germans who were feeling disillusioned and angry.
Rearmament and Expansionism: Hitler began to rearm Germany in violation of the Treaty of Versailles, and pursued a policy of territorial expansion. He annexed Austria in 1938 and then began to make territorial demands on Czechoslovakia. This led to the Munich Agreement in 1938, in which Britain and France agreed to cede parts of Czechoslovakia to Germany. This further emboldened Hitler and made it clear that he was willing to use military force to achieve his goals.
Invasion of Poland: Finally, in September 1939, Germany invaded Poland, which was the trigger for the start of World War II. This invasion was the culmination of Hitler's increasingly aggressive actions, and it led to Britain and France declaring war on Germany.
In summary, the harsh Treaty of Versailles, the rise of Nazism, Hitler's rearmament and expansionism, and the invasion of Poland were all key factors that led to the outbreak of World War II.