By convincing the United States to enter the war, the Allies overwhelmed German forces, pushing them to retreat and eventually surrender on November 11, 1918. Germany, Britain, France, Italy and Russia negotiated and signed the Treaty of Versailles on June 28, 1919, formally ending the war.
By 1917, three years after World War I began, the Allies, made up of Britain, France and Russia, were losing. Outmanned and outmaneuvered, Britain reached out to the United States, which had remained neutral. The United States joined the war after a German U-boat sunk the British ocean liner RMS Lusitania, killing 159 Americans, and after Germany attempted to persuade Mexico to join the war against the United States. This influx of new soldiers gave the floundering Allies the support they needed to gain the upper hand, leaving Germany no choice but to surrender and negotiate a settlement.
By signing the Treaty of Versailles, Germany agreed to limit its army to 100,000 people, pay for its role in the war and let the Allied Nations divide conquered German territories. Though it fought alongside the Allies, the United States opted not to sign the Treaty, objecting to many of its terms, including how much it required Germany to pay for its role in the war. The United States reached its own settlement with Germany.