At the beginning of World War II, the Royal Navy was the strongest navy in the world, with the largest number of warships built and with naval bases across the globe. It had over 15 battleships and battlecruisers, 7 aircraft carriers, 66 cruisers, 164 destroyers and 66 submarines. With a massive merchant navy, about a third of the world total, it also dominated shipping. The Royal Navy fought in every theatre from the Atlantic, Mediterranean, freezing Northern routes to Russia and the Pacific ocean.
In the course of the war the United States Navy grew tremendously as the United States was faced with a two-front war on the seas. By the end of World War II the U.S Navy was larger than any other navy in the world.