President Woodrow Wilson signed the congressional declaration of war against Germany, four days after asking Congress to act following unrestricted German submarine warfare and the intercepted Zimmermann Telegram. America's entry brought fresh troops, industrial might, and financial resources that would tip the balance on the exhausted Western Front within nineteen months.
Wilson had campaigned for reelection in 1916 on the slogan "He kept us out of war," and much of the American public still favored neutrality. But Germany's resumption of unrestricted submarine warfare in early 1917, sinking American merchant ships without warning, combined with the shocking revelation of the Zimmermann Telegram—a secret German proposal urging Mexico to attack the United States—turned public opinion decisively.
The Senate had voted 82 to 6 and the House 373 to 50 in favor of war just days earlier. When Wilson signed the declaration at 1:18 in the afternoon, the United States possessed a standing army of barely 130,000 men, laughably small compared to the millions locked in trench warfare across France and Belgium.
Within a year, American conscription and mobilization transformed that tiny force into an army of over two million men shipped to Europe. Their arrival, alongside American industrial output and loans to exhausted Allied economies, broke the stalemate. The war ended in November 1918, and Wilson's decision reshaped the twentieth century, launching America onto the world stage as a dominant global power.
Key people: Woodrow Wilson