The sinking of the Lusitania
Germany said that it would try not to sink any neutral ships but that mistakes could easily happen. The first so-called mistake wasn't a neutral ship but the British liner Lusitania carrying
ammunition as well as passengers. The Germans sunk it off Ireland in 1915, killing over a thousand people, more than a hundred of them Americans.
Germany agreed to stop sinking passenger liners but reasonably asked that Britain respond by lifting its blockade, which was starving the German people. Britain refused, so safety on the sea was definitely up in the air as the U.S. got ready to vote for president in 1916. Wilson barely squeaked through to reelection on a platform that said, "He kept us out of war," but he made no guarantees for the future.
America enters the Great War
At the beginning of 1917, Wilson made one last, moving speech asking the fighting powers to come to the peace table. Meanwhile, the Germans made a large and dangerous tactical decision. Figuring that it would take the U.S. longer to get to France than it would take Germany to win the war with full submarine warfare, the Germans announced that all bets were off. They were blockaded, and they intended to blockade Britain by sinking any ships headed that way.
The Germans sent a note called the Zimmermann telegram (1917) to Mexico, inviting them to invade the United States with the help of Japan. This harebrained scheme would never have worked, and the Mexicans knew it. Britain, who had been happily reading diplomatic mail from other countries that passed through their island along the transatlantic cable, intercepted the telegram and excitedly showed it to the United States. The Germans had already started sinking ships, and the telegram was the last straw for even peace-loving Wilson. The United States declared war on Germany.