On the eastern front, the slaughter did not immediately degenerate into trench warfare, and the fighting was dominated by Germany. The Germans repulsed the initial Russian attacks, counterattacked, and by 1915 the eastern front had stabilized in Germany’s favor. A staggering 2.5 million Russian soldiers had been killed, wounded, or captured. German armies occupied huge swaths of the Russian empire in central Europe. Yet Russia continued to fight, marking another failure of the Schlieffen Plan.

To govern these occupied territories, the Germans installed a vast military bureaucracy. Anti-Slavic prejudice dominated the mind-set of the occupiers, who viewed the local Slavs as savages and ethnic “mongrels.” About one-third of the civilian population was killed or became refugees under this brutal occupation. In the long run, the German state hoped to turn these territories into German possessions, a chilling forerunner of Nazi policies in World War II.2

The changing tides of war brought neutral countries into the war (see Map 25.3). Italy, a member of the Triple Alliance since 1882, had declared its neutrality in 1914 on the grounds that Austria had launched a war of aggression. Then in May 1915, Italy switched sides to join the Triple Entente in return for promises of Austrian territory. The war along the Italian-Austrian front cost some 600,000 Italian lives.

In October 1914, the Ottoman Empire joined Austria and Germany, by then known as the Central Powers. The following September, Bulgaria followed suit in order to settle old scores with Serbia. The Balkans, with the exception of Greece, were occupied by the Central Powers.

The entry of the Ottomans carried the war into the Middle East. Heavy fighting between the Ottomans and the Russians enveloped the Armenians, who lived on both sides of the border. When in 1915 some Armenians welcomed Russian armies as liberators, the Ottoman government, with German support, ordered a mass deportation of its Armenian citizens from their homeland. In this early example of modern ethnic cleansing, about 1 million Armenians died from murder, starvation, and disease.

In 1915, at the Battle of Gallipoli, British forces tried and failed to take the Dardanelles and Constantinople from the Ottoman Turks. The ten-month-long battle cost the Ottomans 300,000 and the British 265,000 men killed, wounded, or missing.

The British were more successful at inciting the Arabs to revolt against their Ottoman rulers. They bargained with the foremost Arab leader, Hussein ibn-Ali (1856–1931). Controlling much of the Ottoman Empire’s territory along the Red Sea, an area known as the Hejaz (see Map 25.5), Hussein managed in 1915 to win vague British commitments for an independent Arab kingdom. In 1916, Hussein rebelled against the Turks, proclaiming himself king of the Arabs. Hussein was aided by the British officer T. E. Lawrence, who in 1917 helped lead Arab soldiers in a successful guerrilla war against the Turks on the Arabian peninsula.

The British enjoyed similar victories in the Ottoman province of Iraq. British troops occupied Basra in 1914 and captured Baghdad in 1917. In September 1918, British armies and their Arab allies rolled into Syria. This offensive culminated in the entry of Hussein’s son Faisal (FIE-suhl) into Damascus. Arab patriots in Syria and Iraq now expected a large, unified Arab nation-state to rise from the dust of the Ottoman collapse — though they would later be disappointed by the Western powers (see "The Peace Settlement in the Middle East").

The war spread to East Asia and colonial Africa as well. Japan declared war on Germany in 1914, seized Germany’s Pacific and East Asian colonies, and used the opportunity to expand its influence in China. In Africa, colonial subjects of the British and French generally supported the Allied powers and helped local British and French commanders take over German colonies. More than a million Africans and Asians served in the various armies of the warring powers. Large numbers of troops came from the British Commonwealth, a voluntary association of former British colonies.

After three years of refusing to play a fighting role, the United States was finally drawn into the expanding conflict. American intervention grew out of the war at sea and general sympathy for the Triple Entente. At the beginning of the war, Britain and France established a naval blockade to strangle the Central Powers. In early 1915, Germany retaliated with attacks on supply ships from a new weapon, the submarine.

In May 1915, a German submarine sank the British passenger liner Lusitania, claiming more than 1,000 lives, among them 139 U.S. citizens. President Woodrow Wilson protested vigorously. To avoid almost-certain war with the United States, Germany halted its submarine warfare for almost two years.

Early in 1917, the German military command — hoping that improved submarines could starve Britain into submission before the United States could come to its rescue — resumed unrestricted submarine warfare. This gamble failed, and the United States declared war on Germany in April of that year. Eventually the United States tipped the balance in favor of the British, French, and their allies.