During World War I, the German Empire was one of the Central Powers. It began participation in the conflict after the declaration of war against Serbia by its ally, Austria-Hungary. German forces fought the Allies on both the eastern and western fronts. A tight blockade in the North Sea (lasting until 1919) imposed by the Royal Navy reduced Germany's overseas access to raw materials and caused food shortages in the cities, especially in the winter of 1916–17, known as the Turnip Winter.

In the west, Germany sought a quick victory by encircling Paris using the Schlieffen Plan. But it failed due to Belgian resistance, Berlin's diversion of troops, and very stiff French resistance on the Marne, north of Paris. The Western Front became an extremely bloody battleground of trench warfare. The stalemate lasted from 1914 until early 1918, with ferocious battles that moved forces a few hundred yards at best along a line that stretched from the North Sea to the Swiss border.

More wide open was the fighting on the Eastern Front. In the east, there were decisive victories against the Russian army, the trapping and defeat of large parts of the Russian contingent at the Battle of Tannenberg, followed by huge Austrian and German successes. The breakdown of Russian forces – exacerbated by internal turmoil caused by the 1917 Russian Revolution – led to the Treaty of Brest-Litovsk the Bolsheviks were forced to sign on 3 March 1918 as Russia withdrew from the war. It gave Germany control of Eastern Europe.

By defeating Russia in 1917, Germany was able to bring hundreds of thousands of combat troops from the east to the Western Front, giving it a numerical advantage over the Allies. By retraining the soldiers in new storm-trooper tactics, the Germans expected to unfreeze the Battlefield and win a decisive victory before the American army arrived in strength. However, the spring offensives all failed, as the Allies fell back and regrouped, and the Germans lacked the reserves necessary to consolidate their gains.

Food scarcity became a serious problem by 1917. The United States joined with the Allies in April 1917. The entry of the United States into the war – following Germany's declaration of unrestricted submarine warfare – marked a decisive turning-point against Germany. At the end of the war, Germany's defeat and widespread popular discontent triggered the German Revolution of 1918–1919 which overthrew the monarchy and established the Weimar Republic.