At the start of World War II, the Royal Navy was the strongest navy in the world,British and Commonwealth Navies at the Beginning and End of World War 2 Naval-history.net with the largest number of warships built and with naval bases across the globe.Fleets 1939 It had over 15 battleships and battlecruisers, 7 aircraft carriers, 66 cruisers, 164 destroyers and 66 submarines. With a massive merchant navy, a third of the world total, the British also dominated shipping. The Royal Navy fought in every theatre from the Atlantic, Mediterranean, freezing Northern routes to Russia and the Pacific Ocean.

Over 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 by far the largest and most powerful navy in the world with 6,768 ships, including 28 aircraft carriers, 23 battleships, 71 escort carriers, 72 cruisers, 232 submarines, 377 destroyers, and thousands of amphibious, supply and auxiliary ships.

Main navies

+ Naval production during WWII Country Aircraft carriers Figure in parentheses indicates escort carriers built on merchant vessel hulls. BattleshipsNot including ships built prior to the war Cruisers Destroyers Convoy escorts Submarines Merchant tonnage Notes United States 28 (71)1072377 420232 33,993,230 British Empire and Commonwealth of Nations19 (46)55733587526421,000,000 (1939) - 22,000,00018M UK4M CanadaCanada - includes 434 commissioned vessels including cruisers, destroyers, frigates, corvettes and auxiliaries Soviet Union3759150218 Japan30+12 seaplane tenders including 1-400class submarines 1252209 18967 Type D escort ships + 56 Type C escort ships + 29 s + 8 s + 14 s + 4 s 213 4,152,361 Germany 1 Not completedNone completed by the end of the war. Two were in production, and Flugzeugträger B. 4 12 17 22 1,140Wagner, Kennedy, Osborne, and Reyburn. The Library of Congress World War II Companion. pp 202–203. Italy 1 3 6 6 28 1,469,606 Romania 0There were 5 sea-going monitors 0There were 3 old coastguard cruisers 54 proper destroyers plus 1 sea-going torpedo boat 9One frigate, six corvettes, and two escort MTBs 8Including 5 midget submarines 516,000Mark Axworthy-"Third Axis, Fourth Ally-Romanian Armed Forces in the European War, 1941–1945" Poland 43 joined the British and fought throughout the end of World War II 52 joined the British and fought throughout the end of World War II

United States

The United States Navy grew rapidly during World War II from 1941 to 1945, and played a central role in the Pacific theatre in the war against Japan. It also played a major supporting role, alongside the Royal Navy, in the European war against Germany.

The Imperial Japanese Navy (IJN) sought naval superiority in the Pacific by sinking the main American battle fleet at Pearl Harbor, which was built around its battleships. The December 1941 surprise attack on Pearl Harbor did knock out the battle fleet, but it did not touch the aircraft carriers, which became the mainstay of the rebuilt fleet.

Naval doctrine had to be changed overnight. The United States Navy (like the IJN) had followed Alfred Thayer Mahan's emphasis on concentrated groups of battleships as the main offensive naval weapons.Trent Hone, "The Evolution of Fleet Tactical Doctrine in the U.S. Navy, 1922–1941," Journal of Military History (2003) 67#4 pp. 1107–1148 in JSTOR The loss of the battleships at Pearl Harbor forced Admiral Ernest J. King, the head of the Navy, to place primary emphasis on the small number of aircraft carriers.Henry M. Dater, "Tactical Use of Air Power in World War II: The Navy Experience," Military Affairs, Vol. 14, (1950), pp. 192–200 in JSTOR

The U.S. Navy grew tremendously as it faced a two-front war on the seas. It achieved notable acclaim in the Pacific War, where it was instrumental to the Allies' successful "island hopping" campaign. The U.S. Navy fought five great battles with the Imperial Japanese Navy (IJN): the Battle of the Coral Sea, the Battle of Midway, the Battle of the Philippine Sea, the Battle of Leyte Gulf, and the Battle of Okinawa.Samuel Eliot Morison, Two-Ocean War: A Short History of the United States Navy in the Second World War (1963)

By war's end in 1945, the United States Navy had added thousands of new ships, including 18 aircraft carriers and 8 battleships, and had over 70% of the world's total numbers and total tonnage of naval vessels of 1,000 tons or greater.Weighing the U.S. Navy Defense & Security Analysis, Volume 17, Issue 3 December 2001, pp. 259–265.King, Ernest J., USN. "Major Combatant Ships Added to United States Fleet, 7 December 1941 – 1 October 1945", ibiblio.org. US Navy at War 1941–1945: Official Report to the Secretary of the Navy. Retrieved 8 April 2006. At its peak, the U.S. Navy was operating 6,768 ships on V-J Day in August 1945, including 28 aircraft carriers, 23 battleships, 71 escort carriers, 72 cruisers, over 232 submarines, 377 destroyers, and thousands of amphibious, supply and auxiliary ships.

1941–1942 The American war plan was Rainbow 5 and was completed on 14 May 1941. It assumed that the United States was allied with Britain and France and provided for offensive operations by American forces in Europe, Africa, or both. The assumptions and plans for Rainbow 5 were discussed extensively in the Plan Dog memo, which concluded ultimately that the United States would adhere to a Europe-first strategy, making the war against Germany a higher priority than the war against Japan. However President Roosevelt did not approve the plan—he wanted to play it by ear. The Navy wanted to make Japan the top target, and in 1941–1943 the U.S. in effect was mostly fighting a naval war against Japan, in addition to its support for Army landings in North Africa, Sicily and Italy in 1942–1943.Mark A. Stoler, "The 'Pacific-First' Alternative in American World War II Strategy." International History Review 2#3 (1980): 432–452.

U.S. strategy in 1941 was to deter Japan from further advances toward British, Dutch, French and American territories to the south. When the Allies cut off sales of oil to Japan, it lost 90% of its fuel supply for airplanes and warships. It had stocks that would last a year or two. It had to compromise or fight to capture British and Dutch wells to the south. in November 1941, U.S. Army Chief of Staff George Marshall explained the American air war strategy to the press—it was top secret and not for publication: We are preparing for an offensive war against Japan, whereas the Japs believe we are preparing only to defend the Philippines. ...We have 35 Flying Fortresses already there—the largest concentration anywhere in the world. Twenty more will be added next month, and 60 more in January....If war with the Japanese does come, we'll fight mercilessly. Flying Fortresses will be dispatched immediately to set the paper cities of Japan on fire. There won't be any hesitation about bombing civilians—it will be all-out.

Marshall was talking about long-range B-17 bombers based in the Philippines, which were within range of Tokyo. After Japan captured the Philippines in early 1942, American strategy refocused on a naval war focusing on the capture of islands close enough for the intensive bombing campaign Marshall spoke about. In 1944, the Navy captured Saipan and the Mariana Islands, which were within range of the new B-29 bombers.

After its Pearl Harbor victory in early December, the Imperial Japanese Navy (IJN) seemed unstoppable because it outnumbered and outgunned the disorganized Allies—US, Britain, Netherlands, Australia, and China. London and Washington both believed in Mahanian doctrine, which stressed the need for a unified fleet. However, in contrast to the cooperation achieved by the armies, the Allied navies failed to combine or even coordinate their activities until mid-1942. Tokyo also believed in Mahan, who said command of the seas—achieved by great fleet battles—was the key to sea power. Therefore, the IJN kept its main strike force together under Admiral Yamamoto and won a series of stunning victories over the Americans and British in the 90 days after Pearl Harbor.

Outgunned at sea, with its big guns lying at the bottom of Pearl Harbor, the American strategy for victory required a slow retreat or holding action against the IJN until the much greater industrial potential of the US could be mobilized to launch a fleet capable of projecting Allied power to the enemy heartland.

Midway

The Battle of Midway, together with the subsequent Guadalcanal campaign, marked the turning point in the Pacific. "...considered the decisive battle of the war in the Pacific." "Midway was indeed 'the' decisive battle of the war in the Pacific.", p. 166 Between 4–7 June 1942, the U.S. Navy decisively defeated a Japanese naval force that had sought to lure the U.S. carrier fleet into a trap at Midway Atoll. The Japanese fleet lost four aircraft carriers and a heavy cruiser compared to the U.S. Navy's one carrier and a destroyer. After Midway, and the exhausting attrition of the Solomon Islands campaign, Japan's shipbuilding and pilot training programs were unable to keep pace in replacing their losses while the U.S. steadily increased its output in both areas. Military historian John Keegan called the Battle of Midway "the most stunning and decisive blow in the history of naval warfare."Keegan, John. "The Second World War." New York: Penguin, 2005. (275)

Guadalcanal

Guadalcanal, fought from August 1942 to February 1943, was the first major Allied offensive of the war in the Pacific Theater. This campaign saw American air, naval and ground forces (later augmented by Australians and New Zealanders) in a six-month campaign slowly overwhelm determined Japanese resistance. Guadalcanal was the key to control the Solomon Islands, which both sides saw as strategically essential. Both sides won some battles but both sides were overextended in terms of supply lines.

The rival navies fought seven battles, with the two sides dividing the victories. They were: Battle of Savo Island, Battle of the Eastern Solomons, Battle of Cape Esperance, Battle of the Santa Cruz Islands, Naval Battle of Guadalcanal, Battle of Tassafaronga and Battle of Rennell Island. Both sides pulled out their aircraft carriers, as they were too vulnerable to land-based aviation.Samuel Eliot Morison, History of United States Naval Operations in World War II. Vol. 5: The Struggle for Guadalcanal, August 1942–February 1943 (1949)

1943 In preparation of the recapture of the Philippines, the Navy started the Gilbert and Marshall Islands campaign to retake the Gilbert and Marshall Islands from the Japanese in summer 1943. Enormous effort went into recruiting and training sailors and Marines, and building warships, warplanes and support ships in preparation for a thrust across the Pacific, and to support Army operations in the Southwest Pacific, as well as in Europe and North Africa.James C. Bradford, ed., A Companion to American Military History (2010) 1: 205–207, 402–403

In the New Georgia Campaign and the Bougainville Campaign the United States retook the Solomon Islands to the west of Guadalcanal.

1944 The Navy continued its long movement west across the Pacific, seizing one island base after another. Not every Japanese stronghold had to be captured; some, like the big bases at Truk, Rabaul and Formosa, were neutralized by air attack and then simply leapfrogged. The ultimate goal was to get close to Japan itself, then launch massive strategic air attacks and finally an invasion. The US Navy did not seek out the Japanese fleet for a decisive battle, as Mahanian doctrine would suggest; the enemy had to attack to stop the inexorable advance.