Germany retaliated by using its submarines to destroy neutral ships that were supplying the Allies. The formidable U-boats (unterseeboots) prowled the Atlantic armed with torpedoes. A string of attacks on merchant ships followed, culminating in the sinking of the British ship Lusitania by a German U-boat on May 7, 1915. In response to the U-Boat attacks, Allied merchant ships sailed in groups, called convoys, escorted by warships. The convoys were harder for U-Boats to find and attack, but the U-Boats still posed a terrifying threat. By the end of 1917, 3,170 Allied and neutral ships, totaling nearly six million tons, were sunk.
