From the time the United States entered the war, its primary objective was an attack on Germany, presumably by invading France. To do this, control of the sea was essential. Germany opened a U-boat offensive, and in the first four months of 1942 U-boats operating off the east coast of the United States sank 87 ships.

Particularly heavy was the loss in tankers, which carried gasoline from Gulf Coast refineries to cities along the Atlantic coast of the United States. Now the tankers were needed to supply the armed forces as well, and the situation became critical. One solution was the construction of new pipelines to release the tankers for overseas duty. Another was rationing of gasoline in the United States.

To curb the sinkings, the Navy began with sub-chasers, motorboats, and yachts, supplemented by a growing air patrol. Then it added new weapons such as escort carriers, destroyer escorts, and frigates. Submarine detection devices such as radar and sonar were improved. In the first six months of 1942 the number of merchant ships in the Atlantic had declined 12 per cent despite new construction. By October the number remained constant. By the end of the year there was an increase of 5 per cent. The turning point came in the spring of 1943 when escort carriers, destroyer escorts, and long-range patrol bombers became available for convoy operations.

During the last half of 1943 the Germans lost a submarine for every ship they sank. The great invasions of Africa and Europe proved that the Allies had won control of the seas; between May and September of 1943, 62 convoys with more than 3,500 ships crossed between the United States and Great Britain with no losses.