The Atlantic U-boat campaign of World War I pitched German U-boat submarines against Allied ships and merchant vessels at sea. The U-boats were particularly effective in destroying merchant ships delivering vital supplies to Britain. They torpedoed ships on sight, even sinking liners. On 7 May 1915, the liner Lusitania was sunk by a German U-boat. The loss of Allied ships was so extreme (Allied shipping losses spiked at 550,000 tons by May 1917) that from 24 May 1917, all Allied merchant ships were to travel in escorted convoys. Under these arrangements cruisers, troop ships, armed trawlers and torpedo boats, using aerial reconnaissance equipment to detect submarines, would accompany between 10–50 merchant ships. British shipping losses plummeted and there was a small increase in the number of German U-boats lost.