World War II, the most deadly and destructive conflict in history, began with Japan’s surprise attack on Pearl Harbor. The 40 million to 50 million casualties and economic upheavals that followed made it a continuation, after an uneasy 20-year hiatus, of disputes left unresolved by World War I.

The major powers of Europe found themselves divided between the Allied Powers—France, Britain and Russia—and the Central Powers—Germany, Austria-Hungary and Italy. In addition, Japan was pursuing an aggressive policy that put it on a collision course with the Allied Powers.

In the summer of 1914, political leaders and military planners hoped for a quick and decisive victory. The mobilization plans of the armies limited the scope for diplomacy. Moreover, in terms of armament, training, doctrine, discipline and fighting spirit, the German army, known as the Wehrmacht, was considered superior to the Allied forces.

On the battlefields of northern France and Belgium, attritional struggle turned the land into a lunar mud-scape of shell craters and abandoned equipment. For much of the war, the front was a stalemate, and the Allies struggled to overcome it.

In the Pacific, the US Navy severely weakens Japanese naval power. In the East, a series of strategic bombing campaigns devastates Germany. At Stalingrad, the Soviet Red Army pushes German forces back to Berlin. And in North Africa, the Allies invade Vichy French-held Morocco and Algeria to clear Axis troops from Africa. Ultimately, this led to the arrest and expulsion of Benito Mussolini and the defeat of Axis forces in Italy.