The Battle of the Coral Sea, from 4 to 8 May 1942, was a major naval battle between the Imperial Japanese Navy (IJN) and naval and air forces of the United States and Australia. Taking place in the Pacific Theatre of World War II, the battle was the first naval action in which the opposing fleets neither sighted nor fired upon one another, attacking over the horizon from aircraft carriers instead. It was also the first military battle between aircraft carriers.
To strengthen their defensive position in the South Pacific, the Japanese decided to invade and occupy Port Moresby (in the Territory of Papua) and Tulagi (in the southeastern Solomon Islands). The plan, Operation Mo, involved several major units of Japan's Combined Fleet. Two fleet carriers and a light carrier were assigned to provide air cover for the invasion forces, under the overall command of Admiral Shigeyoshi Inoue. The US learned of the Japanese plan through signals intelligence and sent two US Navy carrier task forces and a joint Australian-American cruiser force to oppose the offensive, under the overall command of US Admiral Frank J. Fletcher.
On 3–4 May, Japanese forces invaded and occupied Tulagi, although several supporting warships were sunk or damaged in a surprise attack by the US carrier . Alerted to the presence of enemy aircraft carriers, the Japanese fleet carriers advanced towards the Coral Sea to locate and destroy the Allied naval forces. On the evening of 6 May, the two carrier fleets closed to within but did not detect each other in the darkness. The next day, both fleets launched airstrikes against what they thought were the enemy fleet carriers, but both sides actually attacked other targets. The US sank the Japanese light carrier , and the Japanese sank the destroyer , and damaged the fleet oiler . On 8 May, both sides finally located and attacked the other's fleet carriers, leaving the Japanese fleet carrier damaged, the US fleet carrier Yorktown damaged, and the so critically damaged she was later scuttled.
Both sides having suffered heavy aircraft losses and carriers sunk or damaged, the two forces disengaged and retired from the area. Because of the loss of carrier air cover, Inoue also recalled the Port Moresby invasion fleet. Although the battle was a tactical victory for the Japanese in terms of ships sunk, it has been described as a strategic victory for the Allies. The battle marked the first time since the start of the war that a major Japanese advance had been turned back. More important, the damage to Shōkaku and the aircraft losses of prevented both ships from participating in the Battle of Midway the following month.
Background
Japanese expansion
On 8 December 1941 (7 December US time), Japan declared war on the US and the British Empire, after Japanese forces attacked Malaya, Singapore and Hong Kong as well as the US naval base at Pearl Harbor. In launching this war, Japanese leaders sought to neutralize the US fleet, seize territory rich in natural resources, and obtain strategic military bases to defend their far-flung empire. In the words of the Imperial Japanese Navy (IJN) Combined Fleet's "Secret Order Number One", dated 1 November 1941, the goals of the initial Japanese campaigns in the impending war were to "[eject] British and American strength from the Netherlands Indies and the Philippines, [and] to establish a policy of autonomous self-sufficiency and economic independence."; see also .
To support these goals, during the first few months of 1942, besides Malaya, Japanese forces attacked and took control of the Philippines, Singapore, the Dutch East Indies, Wake Island, New Britain, the Gilbert Islands and Guam, inflicting heavy losses on opposing Allied land, naval and air forces. Japan planned to use these conquered territories to establish a perimeter defense for its empire from which it expected to employ attritional tactics to defeat or exhaust any Allied counterattacks.; ; ; ; .
Shortly after the war began, the Japanese Naval General Staff, under the leadership of Admiral Osami Nagano, recommended an invasion of Northern Australia to prevent Australia from being used as a base to threaten Japan's perimeter defenses in the South Pacific. The Imperial Japanese Army (IJA) rejected the recommendation, stating that it did not have the forces or shipping capacity available to conduct such an operation. At the same time, Vice Admiral Shigeyoshi Inoue, commander of the IJN's Fourth Fleet (also called the South Seas Force) which consisted of most of the naval units in the South Pacific area, advocated the occupation of Tulagi in the southeastern Solomon Islands and Port Moresby in New Guinea, which would put Northern Australia within range of Japanese land-based aircraft. Inoue believed the capture and control of these locations would provide greater security and defensive depth for the major Japanese base at Rabaul on New Britain. The navy's general staff and the IJA accepted Inoue's proposal and promoted further operations, using these locations as supporting bases, to seize New Caledonia, Fiji, and Samoa and thereby cut the supply and communication lines between Australia and the United States.; ; ; ; ; ; ; ; ; ; ; . The Senshi Sōshō does not mention Inoue's role in the decision to invade Port Moresby, only stating it was a product of an agreement between the IJN and IJA in January 1942 ().
In April 1942, the army and navy developed a plan that was titled Operation MO. The plan called for Port Moresby to be invaded from the sea and secured by 10 May. The plan also included the seizure of Tulagi on 2–3 May, where the navy would establish a seaplane base for potential air operations against Allied territories and forces in the South Pacific and to provide a base for reconnaissance aircraft. Upon the completion of MO, the navy planned to initiate Operation RY, using ships released from MO, to seize Nauru and Ocean Island for their phosphate deposits on 15 May. Further operations against Fiji, Samoa and New Caledonia (Operation FS) were to be planned once MO and RY were completed. Because of a damaging air attack by Allied land- and carrier-based aircraft on Japanese naval forces invading the Lae-Salamaua area in New Guinea in March, Inoue requested Japan's Combined Fleet send carriers to provide air cover for MO. Inoue was especially worried about Allied bombers stationed at air bases in Townsville and Cooktown, Australia, beyond the range of his own bombers, based at Rabaul and Lae.; ; ; ; ; ; ; ; ; . The IJA and IJN agreed to wait until the planned operation to occupy Midway and the Aleutians was completed before attacking Fiji and Samoa (). The Senshi Sōshō states IJN troops were also to seize Samarai Island to secure the China Strait through the Louisiades ().
Admiral Isoroku Yamamoto, commander of the Combined Fleet, was concurrently planning an operation for June that he hoped would lure the US Navy's carriers, none of which had been damaged in the Pearl Harbor attack, into a decisive showdown in the central Pacific near Midway Atoll. In the meantime Yamamoto detached some of his large warships, including two fleet carriers, a light carrier, a cruiser division, and two destroyer divisions, to support MO, and placed Inoue in charge of the naval portion of the operation.; ; ; ; ; ; . The carrier was originally assigned to the MO operation. but was replaced by the 5th Carrier Division on 12 April after Inoue complained that one fleet carrier was not sufficient (Lundstrom and Parshall).
Allied response
Unknown to the Japanese, the US Navy, led by the Communication Security Section of the Office of Naval Communications, had for several years enjoyed increasing success with penetrating Japanese communication ciphers and codes. By March 1942, the US was able to decipher up to 15% of the IJN's Ro or Naval Codebook D code (called "JN-25B" by the US), which was used by the IJN for about half of its communications. By the end of April, the US was reading up to 85% of the signals broadcast in the Ro code.; ; . For unknown reasons, the IJN postponed making their scheduled change of the Ro code from 1 April to 1 to 27 May 1942 (Willmott 2002, pp. 21–22; ). The U.S. operated Fleet Radio Units in Washington, D.C., Pearl Harbor and, with the Australians, at Melbourne ().
In March 1942, the US first noticed mention of the MO operation in intercepted messages. On 5 April, the US intercepted an IJN message directing a carrier and other large warships to proceed to Inoue's area of operations. On 13 April, the British deciphered an IJN message informing Inoue that the Fifth Carrier Division, consisting of the fleet carriers and , was en route to his command from Formosa via the main IJN base at Truk. The British passed the message to the US, along with their conclusion that Port Moresby was the likely target of MO.
Admiral Chester W. Nimitz, the new commander of US forces in the Central Pacific, and his staff discussed the deciphered messages and agreed that the Japanese were likely initiating a major operation in the Southwest Pacific in early May with Port Moresby as the probable target. The Allies regarded Port Moresby as a key base for a planned counteroffensive, under General Douglas MacArthur, against Japanese forces in the South West Pacific area. Nimitz's staff also concluded that the Japanese operation might include carrier raids on Allied bases in Samoa and at Suva. Nimitz, after consultation with Admiral Ernest King, Commander in Chief of the United States Fleet, decided to contest the Japanese operation by sending all four of the Pacific Fleet's available aircraft carriers to the Coral Sea. By 27 April, further signals intelligence confirmed most of the details and targets of the MO and RY plans.; ; ; ; ; ; ; ; ; ; ; ; . The British radio interception station was at Colombo on Ceylon (Lundstrom). The U.S. mistakenly believed (in part due to erroneous transliteration of the characters of her name) that Shōhō was a previously unknown fleet carrier, Ryūkaku, with 84 aircraft (). A Japanese prisoner captured at the Battle of Midway informed the U.S. of the correct reading of the carrier's kanji and identified her as actually a light carrier (Lundstrom and Morison, p. 11). The Japanese apparently had not developed cipher codes for several of the islands in the Louisiade Archipelago and thus transmitted the island names in Katakana in the clear, making it easier for the U.S. to decipher the meaning of the messages (Holmes, p. 65). According to Parker (p. 21), MacArthur refused to believe the radio intelligence forecasts of the MO operation and did not acknowledge that the Japanese were attempting to invade Port Moresby until his reconnaissance aircraft actually sighted Japanese ships approaching the Louisiades and New Guinea in the first week of May.