The Treaty of Versailles, which ended World War I, is often seen as a major cause of World War II. The treaty was very harsh on Germany, and it left many Germans feeling resentful and humiliated. The treaty also led to the rise of Adolf Hitler and the Nazi Party, who promised to restore Germany to its former glory.
The Great Depression, which began in 1929, also played a role in the outbreak of World War II. The Depression caused widespread economic hardship in Europe and the United States, and it led to the rise of political extremism in many countries.
The failure of the League of Nations was another factor that contributed to the outbreak of World War II. The League of Nations was an international organization that was created after World War I to prevent future wars, but it was unable to stop the outbreak of World War II.
The Japanese invasion of China in 1937 is also often seen as a major cause of World War II. The invasion led to the outbreak of the Second Sino-Japanese War, and it eventually drew the United States into the war against Japan.
The German invasion of Poland on September 1, 1939, is generally considered to be the start of World War II. The invasion led to Britain and France declaring war on Germany, and the war quickly spread to other countries in Europe.