During World War I, the German Empire was one of the Central Powers that ultimately lost the war. It began participation with 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, although German territory itself remained relatively safe from widespread invasion for most of the war.

Germans responded to the beginning of war during 1914 with the same general enthusiasm as did many people of other countries of Europe; this enthusiasm is known as the Spirit of 1914. The German government, dominated by the economic and political elite, thought of the war as a way to end Germany's disputes with neighbors and rivals like France, the United Kingdom, and Russia. The beginning of war was thus presented in authoritarian Germany as the chance for the nation to secure "our place under the sun" as the Kaiser Wilhelm II put it, which was readily supported by the prevailing nationalism of the public. The Kaiser and the German establishment hoped the war would unite the public behind the monarchy, and lessen the threat posed by the dramatic growth of the Social Democratic Party of Germany, which had been the most vocal critic of the Kaiser in the Reichstag before the war. Despite their history, the Social Democratic Party of Germany ended its differences with the government and abandoned its principles of internationalism to support the war effort.

It soon became apparent that Germany was not prepared for a war lasting more than a few months. At first, little was done to regulate the economy for a wartime footing, and the German war economy would remain badly organized throughout the war. Germany depended on imports of food and raw materials, which were stopped by the British blockade of Germany. Food prices were first limited, then rationing was introduced. The winter of 1916/17 was called "turnip winter". During the war, about 750,000 German civilians died from malnutrition .
Even more died after the war, as the Allied blockade was not ended until the summer of 1919.

Enthusiasm began to decrease as the enormous numbers of casualties began returning from the first battles. As the human cost increased during battles at the Marne, Verdun, the Somme, and at Ypres in the west, and in Poland and Galicia in the East, a grimmer and grimmer attitude began to prevail amongst the general population. Morale was helped by victories against Serbia, Greece, Italy, and Russia which made great gains for the Central Powers. Morale was at its greatest since 1914 at the end of 1917 and beginning of 1918 with the defeat of Russia following her rise into revolution, and the German people braced for what Ludendorff said would be the "Peace Offensive" in the west. In one of the bloodiest series of battles in history from March to August, Ludendorff's plans were thwarted by the combined Allied efforts and Germany's last chance to win the war was lost.

By September 1918, the Central Powers were exhausted from fighting, and the Allies had won the support of American forces. Even though the eastern front was hundreds of miles away from the borders of the Reich, an invasion of the Rhineland on the western front was possible. The hunger and popular dissatisfaction with the war precipitated uprisings and an attempted revolution throughout Germany, deposing the Kaiser and creating the historical motive for far-right German nationalists to later develop the Dolchstoßlegende. By the end of 1918, Germany had signed the Armistice, the Kaiser had abdicated, and the Empire had been replaced by the Weimar Republic.