The Second World War was a global war that lasted from 1939 to 1945 and involved the majority of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis. The war caused 50 million to over 70 million fatalities, making it the deadliest conflict in human history.
There were many causes of the Second World War, but the most significant were the aggressive actions of Nazi Germany, led by dictator Adolf Hitler, and the failures of the international community to effectively address those actions.
One of the main causes of the war was the Treaty of Versailles, which ended the First World War and imposed harsh penalties on Germany. The treaty required Germany to disarm, make significant territorial concessions, and pay large reparations to the Allied powers. Many Germans saw the treaty as a betrayal and resented the penalties imposed on their country.
Hitler and the Nazi Party came to power in Germany in 1933 and pursued a policy of rearmament and expansion. Hitler withdrew Germany from the League of Nations and pursued a policy of territorial expansion, annexing Austria in 1938 and the Sudetenland region of Czechoslovakia in 1938. These actions were met with little resistance from the international community.
In 1939, Hitler invaded Poland, which led to the United Kingdom and France declaring war on Germany. Hitler then invaded Denmark, Norway, Belgium, the Netherlands, and France, and the war spread to Africa, Asia, and the Soviet Union.
The failure of the international community, particularly the League of Nations, to take effective action in response to Hitler's aggressive actions was a significant factor in the outbreak of the war. The League of Nations was unable to effectively address the actions of aggressive powers, and this failure to uphold international law and deter aggression contributed to the outbreak of the war.
Other factors that contributed to the outbreak of the war include the failure of appeasement, the desire for territorial expansion by certain powers, and the complex system of alliances that existed at the time.
Overall, the Second World War was the result of a complex combination of factors, but the aggressive actions of Nazi Germany and the failures of the international community to address those actions were the most significant causes of the war.