By the start of 1917, every major army had suffered losses that would have seemed unthinkable in 1914, and no side was close to victory. That year broke the deadlock in a way no one had planned: one great power left the war in revolution, and another entered it from across the Atlantic.
Armies under strain
In April 1917 the French general Robert Nivelle promised a breakthrough on the Chemin des Dames ridge. It failed with heavy losses, and after the failed Nivelle offensive, many French soldiers refused to attack, though most still agreed to hold their trenches. The new commander, Philippe Pétain, restored order with a mix of punishment and reform: hundreds of soldiers were sentenced to death and a few dozen were shot, but leave became more regular, food improved and pointless attacks stopped. That summer and autumn the British attacked near Ypres at Passchendaele, where rain turned the battlefield into a swamp in which men and horses drowned. In October the Italian army collapsed at Caporetto under an Austro-German attack, though it rallied further back.
Russia: from tsar to Bolsheviks
Russia's war was going worst of all. Millions of soldiers had been killed, wounded or captured, food and fuel were short in the cities, and Tsar Nicholas II had made himself commander in chief, so every defeat became his own. In March 1917 (February in the old Russian calendar), strikes and bread protests in Petrograd, joined by mutinous soldiers, forced the tsar to abdicate, ending three centuries of Romanov rule. Women workers marking International Women's Day were among the first in the streets.
A Provisional Government of liberals and moderate socialists took power, promising elections and civil rights, but it kept fighting the war. It shared power uneasily with the Petrograd Soviet, a council of workers' and soldiers' delegates. Germany, hoping to weaken Russia further, let the Bolshevik leader Vladimir Lenin travel home from Switzerland across German territory. His promise of "peace, land and bread" spoke to exhausted soldiers and peasants. In November 1917 (October in the old calendar) the Bolsheviks seized power in Petrograd.
Lenin's government asked for peace. In the Treaty of Brest-Litovsk, signed in March 1918, Russia gave up Poland, the Baltic lands, Finland and Ukraine, territory holding roughly a third of its prewar population and much of its industry and coal. Russia then fell into a civil war that killed millions more. Germany could now move large forces to the west, though it had to keep many troops in the east to hold its new gains.
America enters
The United States had stayed neutral, and President Woodrow Wilson won reelection in 1916 partly because he had kept the country out of war. But American banks and factories traded heavily with the Allies. In February 1917 Germany resumed unrestricted submarine warfare, sinking without warning any ship in the waters around Britain, including neutral American ships. German leaders gambled that they could starve Britain into surrender before American troops arrived in strength. Britain countered the U-boats by sailing merchant ships in convoys under naval escort, and losses fell sharply.
Then British code breakers handed Washington a decoded message. The Zimmermann Telegram proposed a German alliance with Mexico, offering it Texas, New Mexico and Arizona if the United States went to war with Germany. Published in March 1917, it outraged Americans. On 2 April Wilson asked Congress for war, declaring that "The world must be made safe for democracy." The United States declared war on 6 April 1917. Wilson insisted that his country was an "associated power," not a formal ally of Britain and France.
American troops arrived slowly at first, but under General John J. Pershing about 2 million American soldiers were in France by November 1918. The United States also supplied loans, food and ships. Their arrival in great numbers in 1918, when the other armies were exhausted, made it clear that time was on the Allies' side.
Worked example: Germany's gamble
Put yourself in the German high command in January 1917. You know that unrestricted submarine warfare will probably bring the United States into the war. Your naval experts calculate that sinking a certain tonnage of shipping each month will force Britain to seek peace within about six months, long before American armies can be trained and shipped. On paper, the bet looks rational.
It failed for reasons the planners discounted. Convoys cut the sinkings, America mobilized faster than expected, and the gamble threw away the chance of a negotiated peace with American help. The lesson is a common one in history: plans built on optimistic numbers and short timelines often ignore how opponents adapt. When leaders promise that a risky step will pay off quickly, it is worth asking what happens if it does not.