Unrestricted Submarine Warfare

The policy of sinking merchant and neutral ships without warning, abandoning the prize rules that required stopping and searching a vessel and providing for crew safety. Imperial Germany declared it on 1 February 1917, betting that U-boats could starve Britain out before the United States could act. April 1917 saw about 860,000 gross tons of Allied and neutral shipping sunk, the campaign’s peak. The sinkings, with the Zimmermann Telegram, brought the United States into the war on 6 April 1917; the Allied convoy system then cut the loss rate.