Wilson's initial hope that America could be "impartial in thought as well as in action" was soon compromised by Germany's attempted quarantine of the British Isles. Britain was one of America's closest trading partners, and tension arose between the United States and Germany when several U.S. ships traveling to Britain were damaged or sunk by German mines.
In February 1915, Germany announced unrestricted warfare against all ships, neutral or otherwise, that entered the war zone around Britain. One month later, Germany announced that a German cruiser had sunk the William P. Frye, a private American vessel that was transporting grain to England when it disappeared. President Wilson was outraged, but the German government apologized and called the attack an unfortunate mistake.
In early May 1915, several New York newspapers published a warning by the German embassy in Washington that Americans traveling on British or Allied ships in war zones did so at their own risk. The announcement was placed on the same page as an advertisement for the imminent sailing of the British-owned Lusitania ocean liner from New York to Liverpool. On May 7, the Lusitania was torpedoed without warning by a German submarine just off the coast of Ireland. Of the nearly 2,000 passengers, 1,201 were killed, including 128 Americans.
It was revealed that the Lusitania was carrying about 173 tons of war munitions for Britain, which the Germans cited as further justification for the attack. The United States eventually sent three notes to Berlin protesting the action, and Germany apologized and pledged to end unrestricted submarine warfare. In November, however, a U-boat sunk an Italian liner without warning, killing 272 people, including 27 Americans. Public opinion in the United States began to turn irrevocably against Germany.
In late March, Germany sunk four more U.S. merchant ships, and on April 2, President Wilson appeared before Congress and called for a declaration of war against Germany. On April 4, the Senate voted 82 to six to declare war against Germany. Two days later, the House of Representatives endorsed the declaration by a vote of 373 to 50, and America formally entered World War I.
On June 26, the first 14,000 U.S. infantry troops landed in France to begin training for combat. After four years of bloody stalemate along the Western Front, the entrance of America's well-supplied forces into the conflict was a major turning point in the war. By the time the war finally ended on November 11, 1918, more than 2 million American soldiers had served on the battlefields of Western Europe, and some 50,000 of these men had lost their lives.