During the early stage of World War 2, the German Navy's "U-Boat" submarines posed a great threat to the Allied Forces in the North Atlantic Ocean. To counter them, new destroyers were built to escort transport vessels, as well as escort carriers that could provided constant air protection to the convoys for the length of their journey across the Atlantic. This type of aircraft carrier was a modification from certain merchant vessels, and the U.S. Navy made an extensive development and use of them for convoy escort and aircraft transportation.