Throughout World War I, Germany used its fleet of 351 unterseeboote (colloquially called U-boats in English) to wage submarine warfare against Allied ships.

Facing an overwhelming naval blockade by the British, the Germans responded on Feb. 4, 1915 by declaring the waters around the British Isles a war zone.

While the U-boats had limited success against nimble British warships, merchant vessels and civilian ships operating in the “war zone” were ripe targets for torpedoes.

The sinking of civilian vessels such as the R.M.S. Lusitania ultimately caused the United States to join the war on the side of the Allies.