World War II, which lasted from 1939 to 1945, was one of the most significant conflicts in human history. This global war involved over 30 countries and resulted in an estimated 70-85 million fatalities, making it the deadliest conflict ever.

The causes of World War II are complex, including factors like the Treaty of Versailles, which left Germany economically devastated. Additionally, the rise of totalitarian regimes in Germany, Italy, and Japan fueled ambitions for territorial expansion. The invasion of Poland by Nazi Germany on September 1, 1939, marked the official beginning of the war.

The war witnessed major battles such as D-Day, the Battle of Stalingrad, and the Pacific Campaign. Allied forces, comprising the United States, Soviet Union, and Great Britain, fought against the Axis powers, primarily Germany, Japan, and Italy. The conflict saw the introduction of modern warfare technologies, including tanks, aircraft, and naval fleets.

One of the most significant events was the Holocaust, where millions of Jews and other minorities were systematically exterminated. The war concluded with the unconditional surrender of Germany in May 1945 and Japan in September 1945, following the dropping of atomic bombs on Hiroshima and Nagasaki.

The aftermath of World War II reshaped the world order and led to the establishment of the United Nations, aiming to prevent future global conflicts. The lessons learned from this devastating war continue to influence international relations today.