British merchant seaman take to their lifeboats after their ship is torpedoed.

Early in World War I, both Germany and Great Britain staged naval blockades in an attempt to cut off the other side’s economic and supply lifelines. But it was the Germans who soon grew frustrated with British counter-tactics, which included arming their merchant ships, concealing them under the flags of neutral nations, and ordering captains to attempt to ram German U-Boats that challenged them at sea. Thus, in February 1915, the Germans decided to raise the ante. They declared that the waters around the British Isles were now a war zone, in which they would sink Allied merchant ships on sight. In addition, the Germans noted, vessels and passengers from neutral countries should avoid those waters as well, for their own safety.

That part of the warning clearly was aimed at the United States, Great Britain’s most important trading partner. President Woodrow Wilson’s administration responded with a stern message that the U.S. would hold Germany to "a strict accountability" for any harm to Americans or damage to U.S. ships. But when the Germans sank the British steamer Falaba, killing an American passenger, and then damaged two American tankers, Wilson did nothing. It would take another attack, one of catastrophic proportions, to push the U.S. in a direction that eventually would lead to its entry into the war.