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.
Germany had suspended its unrestricted U-boat attacks in 1915 after the United States protested the sinking of the Lusitania ocean liner and other unarmed vessels with Americans aboard.
Three days after the renewal of unlimited submarine warfare in 1917, the United States broke diplomatic relations with Germany, and hours after that the American merchant liner Housatonic was sunk by a U-boat. None of the 25 Americans on board were killed, and a British steamer later picked them up.
Within two months, U.S. President Woodrow Wilson and an outraged Congress declared war, giving the Allies a major boost in morale and reinforcements.