The world had entered a period of unprecedented economic, social and political disruption. It was a time of reordering world power dynamics, as new nations sought to safeguard their interests by entering into a network of alliances with each other and the established powers.

Amid increasing nationalism, militarism and imperial rivalry, European powers had been building up their militaries for years. They were also vying for foreign investments and territory in the Balkans, Africa and Asia. This created tensions between the great powers, which would eventually lead to a global war that killed more than 9 million people.

The acrimony between the great powers led to two sets of opposing alliances, with Germany and Austria-Hungary on one side and Russia, France and Britain on the other. Throughout the course of the war, some countries would join or leave both sides, and many more would change sides as events in the Balkans, East Asia and elsewhere shifted the balance of forces on all fronts.

By September 1939, the Allies were far superior in industrial resources, population and military manpower, but the German military—known as the Wehrmacht—was the most efficient and effective fighting force of its size in the world. The Allies had been able to hold off the Axis until May 1943, when victory in North Africa enabled them to link up near Tunis and push the German army into retreat. This paved the way for an Allied invasion of Berlin and the eventual surrender of the Axis in November 1945.