The first few months of 1942 saw Japan take control of islands throughout the Central and South Pacific, including the Philippines and Guam, as part of its plan to form a defensive perimeter to protect its empire and blunt Allied counterattacks.
By mid-April, Tokyo was ready to turn its attention to Port Moresby in what is now southeastern Papua New Guinea and at Tulagi in the central Solomon Islands. Establishing air bases there would provide better defensive positions, put Japanese planes within range of Australia, and be jumping-points for further advances, which would allow Japan to secure more resources and to cut supply and communications lines between Australia and the US.
The Japanese campaign, called Operation Mo, took shape in April, with Japan planning to capture Tulagi in early May, followed by the seizure of Port Moresby about a week later. But the US Navy had penetrated Japanese codes and was able to glean details of the operation from Japanese naval movements.
By April, British and US intelligence had a general idea of where and when the attack would come. The US sent a two carriers to counterattack, setting up what would become the Battle of the Coral Sea — the first naval battle in which aircraft carriers engaged each other and the combatants did not come within sight of each other, yielding an Allied victory that set the stage for Japan's defeat.
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Japan, concerned about the buildup of US forces in Australia, thought occupying Tulagi and setting up a seaplane base there would give it an advantage in the Coral Sea.
With considerable intelligence about Japanese plans, US Navy Adm. Chester Nimitz, commander of US forces in the Central Pacific, ordered task forces built around the carriers Lexington and Yorktown to the area at the beginning of May, where they would rendezvous with US and Australian warships.
Source: Barrett Tillman, "On Wave and Wing: The 100 Year Quest to Perfect the Aircraft Carrier"; Royal Australian Navy official history
The two US task forces met on May 1, with Rear Adm. Frank Jack Fletcher taking overall command.
After refueling, Fletcher took the Yorktown into the Coral Sea to search for the Japanese. Late on May 3, he got intelligence about Japanese landings on Tulagi and the next morning launched strikes against the remnants of the landing force, catching them by surprise and sinking a destroyer and several minesweepers and damaging other vessels.
A second strike destroyed several of the aircraft the Japanese had based there, but construction on the base continued. Three US aircraft were downed, but all four fliers were rescued.
The main Japanese invasion force also left Rabaul, in New Britain, for Port Moresby, on May 4, taking a longer route to the east, raising the chance of an encounter with US naval forces led by Fletcher.
Source: Barrett Tillman, "On Wave and Wing: The 100 Year Quest to Perfect the Aircraft Carrier"
On May 5, the Yorktown task force retired southward, looking to reunite with the Lexington task force and refuel.
The main Japanese force, consisting of two fleet aircraft carriers, was sailing southeastward along the Solomon Islands, headed toward the Coral Sea. Each side knew of the other's existence, but not their strength or location. A Yorktown patrol aircraft shot down a Japanese aircraft that morning, which likely prevented the Japanese from discovering more about the Allied force's position.
On May 6, the two US task forces met up. Their combined strength was two carriers, eight cruisers, and 13 destroyers — they also had 130 carrier-based planes. The main Japanese force continued moving south toward the Coral Sea. B-17 bombers based in Australia mounted an unsuccessful attack on the Japanese force, but the two naval forces were not in contact that day.
Source: Australian War Memorial; Barrett Tillman, "On Wave and Wing: The 100 Year Quest to Perfect the Aircraft Carrier"
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