Amid growing concern about Germany’s territorial ambitions, Britain declares war on Germany. German infringements of Belgian neutrality, as well as the country’s own strategic position, tipped the scale toward war; Britain entered to protect its sea routes with the United States and reassert its imperial power. The Allied powers had greater overall demographic, industrial and military resources than the Central Powers; they also enjoyed easier access to oceans for trade with neutral countries, including the United States.

The Allied offensive of 1917 shattered the defensive line known as “No Man’s Land,” which separated opposing trenches. Artillery bombardments continued to pound the front, while new offensive weapons, such as poison gases and tanks (see below), were used to help break through the trenches. Casualties were high. Many men died of gangrene, trench foot and disease from poor hygiene and malnutrition, while others were psychologically scarred by prolonged exposure to the terrifying sounds and sights of battle.

By the end of the war, the major belligerents—Germany, Austria-Hungary, Russia and the Ottoman Empire—were all defeated; their empires were broken up. The Treaty of Versailles imposed enormous war reparations on the losing nations and gave vast tracts of land to the victors. Resentment over these terms contributed to the rise of Adolf Hitler and his neofascist party, which ultimately led to World War II. The resulting global conflict ended with the total victory of the Allied forces, and the founding of the League of Nations to foster international cooperation to prevent future conflicts.