On October 20, 1918, the German Empire ended its program of unrestricted submarine warfare.  Off and on through out the war, German U-Boats had attacked shipping destined for Allied countries without warning.  This reduced the risk to the German boats and crews, but increased the loss of life on targeted ships.  The resumption of the tactic in 1917 was a calculated move to knock Great Britain out of the war before the United States could mass sufficient forces after they declared war.

In four years, German submarines sank over 4000 Allied and neutral ships, with a tonnage in excess of 8,000,000 tons.  Losses to the German fleet was 178 U-Boats and about 5000 sailors.