U-boats submarine warfare RMS Lusitania cruiser rules unrestricted submarine warfare sea lanes Germany Britain United States failed
German U-boats attempted to cut the supply lines between North America and Britain. The nature of submarine warfare meant that attacks often came without warning, giving the crews of the merchant ships little hope of survival. The United States launched a protest, and Germany changed its rules of engagement. After the sinking of the passenger ship RMS Lusitania in 1915, Germany promised not to target passenger liners, while Britain armed its merchant ships, placing them beyond the protection of the "cruiser rules", which demanded warning and movement of crews to "a place of safety" (a standard that lifeboats did not meet). Finally, in early 1917, Germany adopted a policy of unrestricted submarine warfare, realising the Americans would eventually enter the war. Germany sought to strangle Allied sea lanes before the United States could transport a large army overseas, but after initial successes eventually failed to do so.
