On January 31, 1917, Germany announces the renewal of unlimited submarine warfare in the Atlantic, and German torpedo-armed submarines prepare to attack any and all ships, including civilian passenger carriers, said to be sited in war-zone waters. Three days later, the United States broke diplomatic relations with Germany, and just hours after that the American liner Housatonic was sunk by a German U-boat. None of the 25 Americans on board were killed, and all were later picked up by a British steamer.
When World War I erupted in 1914, President Woodrow Wilson pledged neutrality for the United States, a position that the vast majority of Americans favored. Britain, however, was one of America's closest trading partners, and tension soon arose between the United States and Germany over the latter's attempted quarantine of the British isles. Several U.S. ships traveling to Britain were damaged or sunk by German mines, and 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.
The Germans' most formidable naval weapon was the U-boat, a submarine far more sophisticated than those built by other nations at the time. The typical U-boat was 214 feet long, carried 35 men and 12 torpedoes, and could travel underwater for two hours at a time. In the first few years of World War I, the U-boats took a terrible toll on Allied shipping.
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 of the imminent sailing of the British-owned Lusitania ocean liner from New York to Liverpool. On May 7, the Lusitania was torpedoed without warning just off the coast of Ireland. Of the 1,959 passengers, 1,198 were killed, including 128 Americans.
The German government maintained that the Lusitania was carrying munitions, but the U.S. demanded reparations and an end to German attacks on unarmed passenger and merchant ships. In August, Germany pledged to see to the safety of passengers before sinking unarmed vessels but in November 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 1917, Germany, determined to win its war of attrition against the Allies, announced the resumption of unrestricted warfare. The United States broke off relations with Germany, and on February 22 Congress passed a $250 million arms appropriations bill intended to make the United States ready for war. Two days later, British authorities gave the U.S. ambassador to Britain a copy of the "Zimmermann Note," a coded message from German foreign secretary Arthur Zimmermann to Count Johann von Bernstorff, the German ambassador to Mexico. In the telegram, intercepted and deciphered by British intelligence, Zimmermann stated that, in the event of war with the United States, Mexico should be asked to enter the conflict as a German ally. In return, Germany promised to restore to Mexico the lost territories of Texas, New Mexico, and Arizona. On March 1, the U.S. State Department published the note, and American public opinion was galvanized 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. When the war finally ended, on November 11, 1918, more than two million American soldiers had served on the battlefields of Western Europe, and some 50,000 of these men had lost their lives.