Just three days after U.S. President Woodrow Wilson's speech of February 3, 1917—in which he broke diplomatic relations with Germany and warned that war would follow if American interests at sea were again assaulted—a German submarine torpedoes and sinks the Anchor Line passenger steamer California off the Irish coast.
The SS California departed New York on January 29 bound for Glasgow, Scotland, with 205 passengers and crewmembers on board. Eight days later, some 38 miles off the coast of Fastnet Island, Ireland, the ship's captain, John Henderson, spotted a submarine off his ship's port side at a little after 9 a.m. and ordered the gunner at the stern of the ship to fire in defense if necessary. Moments later and without warning, the submarine fired two torpedoes at the ship. One of the torpedoes missed, but the second torpedo exploded into the port side of the steamer, killing five people instantly. The explosion of the torpedo was so violent and devastating that the 470-foot, 9,000-ton steamer sank just nine minutes after the attack. Despite desperate S.O.S. calls sent by the crew to ensure the arrival of rescue ships, 38 people drowned after the initial explosion, for a total of 43 dead.
This type of blatant German defiance of Wilson's warning about the consequences of unrestricted submarine warfare, combined with the subsequent discovery and release of the Zimmermann telegram—an overture made by Germany's foreign minister to the Mexican government involving a possible Mexican-German alliance in the event of a war between Germany and the U.S.—drove Wilson and the United States to take the final steps towards war.
On April 2, Wilson went before Congress to deliver his war message; the formal declaration of U.S. entrance into the First World War came four days later.
Taken from: http://www.history.com/this-day-in-history/german-sub-sinks-us-passenger-ship-california [06.02.2015]