World War II was a global conflict that took place between 1939 and 1945. It was the deadliest conflict in human history, with an estimated 50-85 million fatalities worldwide. The war involved the majority of the world's nations, including all of the great powers, organized into two opposing military alliances: the Allies (primarily composed of the United Kingdom, France, and the Soviet Union) and the Axis (primarily composed of Germany, Italy, and Japan).
The war had its roots in the Treaty of Versailles, which ended World War I, and the global economic crisis of the 1930s. Adolf Hitler rose to power in Germany in 1933, and the country rapidly rearmed and began its expansion through Europe, violating the terms of the Treaty of Versailles. In 1939, Germany signed a non-aggression pact with the Soviet Union and invaded Poland, which prompted Britain and France to declare war on Germany. Over the next several years, the Axis powers conquered much of Europe and launched a massive invasion of the Soviet Union.
In 1941, Japan launched a surprise attack on the US naval base at Pearl Harbor, drawing the United States into the war. The tide began to turn against the Axis powers in 1942, with the Allies launching successful offensives in North Africa and the Soviet Union, and the US defeating Japan in several key battles in the Pacific. In 1944, the Allies launched the D-Day invasion of France, opening up a second front in Europe and leading to the eventual liberation of France and other countries. The war ended in 1945, with the unconditional surrender of Germany in May and the dropping of atomic bombs by the US on the Japanese cities of Hiroshima and Nagasaki in August, which led to Japan's surrender.
The aftermath of World War II was significant, with the formation of the United Nations and the establishment of the United States and the Soviet Union as superpowers. The war also resulted in the Holocaust, a genocide in which approximately six million Jews and other minority groups were murdered by the Nazi regime. The legacy of World War II continues to shape the world today, and the conflict is widely studied and remembered as a defining moment in world history.