 By the spring of 1917 , the unrestricted submarine warfare campaign — which Germany had launched at the beginning of February — had so succeeded that the entire Allied war effort was endangered . Strong reinforcements to the Allied antisubmarine forces were desperately needed to avert defeat and needed at once . In response to a request from the Royal Navy for the service of American antisubmarine warfare ships in European waters , the United States Navy began sending destroyers eastward across the Atlantic . 
