Introduction to World War II (1939–1945)

World War II (1939–1945) was the deadliest, most costly, most widespread war in human history. The war saw the decline of the European colonial powers that had dominated global politics for the previous four centuries and the rise of the United States and the Soviet Union as superpowers. Tremendous advances in science, medicine, and military technology were made over the course of the conflict—from antibiotics to the atomic bomb—and governments spent unprecedented sums of money on research and development in the name of victory.

World War II spurred the complete mobilization of not just science but also industry, national economies, and the civilian labor force. In fact, the majority of deaths in World War II were non-military. In addition to famine and disease, civilians in war-ravaged areas found themselves deliberately targeted by hostile powers using methods ranging from strategic bombing to cold-blooded genocide in the notorious Nazi death camps. In all, more than sixty million people—soldiers and civilians—would lose their lives during World War II. Untold millions more were permanently displaced.

The roots of the conflict lay in the resolution of World War I, when Germany was placed under severe economic and military sanctions and forced to pay war reparations to the victorious powers. Meanwhile, Italy and Japan, although sharing in the victory of the Allies, felt they were not adequately rewarded for their contribution to the war effort. In World War II, these three nations and their allies, known collectively as the Axis powers, would wage war for nearly six years against an increasing number of enemies—the Allies.

The war was in many ways a clash of political and economic ideologies, pitting Western democracy, in an uneasy alliance with Soviet Communism, against imperialist and fascist aggression. The initial stages of the war were marked by stunning Axis victories in Europe and Asia—Germany found itself master of nearly all of Europe by 1941, while Japan, through a series of surprise attacks and lightning campaigns, controlled a vast Pacific realm by 1942.

That year would prove to be the turning point of the war. Germany overextended itself by invading the Soviet Union, while Japan soon found itself on the defensive after bringing the industrial and economic powerhouse of the United States into the conflict. Over the next three years, the Allies would gradually force the Axis powers to retreat over their newly acquired territories, driving them back towards their homelands.

By 1945, the Axis powers were on the verge of collapse. Italy had surrendered in 1943, and most of Germany's minor allies were similarly out of the fight or had even started fighting on the side of the Allies. Yet Germany and Japan kept fighting, spurred on by Allied calls for an unconditional surrender and the threat of retribution that policy implied.

In the end, the measures required to defeat the Axis powers would prove extreme—Germany held out until it was nearly completely overrun, crushed beneath millions of invading soldiers streaming in from the Allied nations. Japan surrendered three months later after suffering tens of thousands of civilian deaths in two devastating moments: the detonation of two atomic bombs at Hiroshima and Nagasaki.