When the U.S. entered World War I in April 1917, America’s sailors were immediately forced to engage in the utterly new realm of anti-submarine warfare waged on, below and above the seas by a variety of small ships and the new technology of airpower. The U.S. Navy substantially contributed to the safe trans-Atlantic passage of a two million man Army that decisively turned the tide of battle on the Western Front even as its battleship division helped the Royal Navy dominate the North Sea. Thoroughly professionalized, the Navy of 1917-18 laid the foundations for victory at sea twenty-five years later.