Why did the United States enter World War I, and what effect did its entry have on the war? Also, what was the state of the war before and after the U.S. involvement?
When World War I broke out in Europe in 1914, Woodrow Wilson was eager to keep the U.S. out of the fighting. He ran for re-election in 1916 with the slogan "he keep us out of war," and his message had a great deal of popular support. A current of isolationism was sweeping the United States, in part because people felt that bankers were eager to enter the war merely to make money. People also saw the conflict as a European affair that did not involve the United States. Many Americans of German descent were not eager to fight against Germany, and many Americans of Irish descent did not support fighting alongside England.
What dragged the U.S. into the war was in part unrestricted German submarine warfare, including the attack on the British ship the Lusitania in 1915 that resulted in over 1,000 deaths, including those of Americans. After the Sussex, a French passenger ship, was torpedoed in 1916, the Germans pledged not to sink any more passenger ships but did not stick to their word. In addition, in 1917, the British intercepted the Zimmermann Telegram, sent from the German Foreign Office to Mexico proposing an alliance between Germany and Mexico if the U.S. entered the war against Germany. The revelation of this telegram helped sway American public opinion in favor of entering the war. Finally, the removal of Russia from the Allies fighting with Great Britain made entering the war more enticing for Americans, who did not want to fight alongside the troops of the Russian tsar. Wilson decided to declare war in April of 1917 with the idea of making the world "safe for democracy."
Before the U.S. entered the war, the Western Front had largely settled into a stalemate, though the British naval blockade of Germany was starting to have an effect on Germany. The Germans mounted an offensive along the Western Front in the spring of 1918, and Wilson sent troops to the Western Front to help the Allies. The addition of one million fresh American troops by August of 1918 turned the tide in the Allies' favor. The allies led a counteroffensive at Sommes that quickly turned the war towards the eventual victory of the Allies. The armistice was signed on November 11, 1918.