The U.S. Navy obtained the Japanese invasion plans through intelligence reconnaissance. In the deployment area near Rabaul, three aircraft carriers, two to three battleships, three heavy cruisers and two light cruisers, 16 destroyers, one submarine tender, six submarines, and several smaller units were identified. A large-scale Japanese operation was in the offing.
After the U.S. air attacks on the Japanese bases on Lae and Salamaua on 10 March 1942, Task Force FOX, consisting of the aircraft carrier USS Yorktown as well as three heavy cruisers and six destroyers, remained in the Coral Sea area of operations, while Task Force BAKER with the carrier USS Lexington returned to Pearl Harbor. On 16 April she received orders from the high command to sail for Christmas Island. While underway there, however, the operational order was revised and a course to the Coral Sea was ordered.
On 1 May, the two task forces met and Rear Admiral Frank Jack Fletcher, commander of Task Force FOX, assumed command. The task force now consisted of the two carriers and eight cruisers, including two from the Australian Navy. A short time later, a reconnaissance plane from the Yorktown sighted a surfaced Japanese submarine about 60 kilometers from the fleet. It could probably be sunk by depth charges from three requested fighter planes, but intercepted radio transmissions indicated that the position of the US units had still been relayed to the Japanese.
The next day, Fletcher received intelligence reports suggesting that an enemy advance toward Port Moresby was imminent. Fletcher responded by taking a northerly course to arrive in the area of operations in time. BAKER Group had not yet completed fuel pickup and was ordered to follow on the night of 4 May.
Tulagi
When Japanese troops attempted to land on Tulagi on 3 May to establish a small air base there, Task Force FOX with the Yorktown launched an air attack from the north on the morning of 4 May against the Japanese landing operation. The destroyer Kikuzuki was badly damaged in Halavo Bay (Florida Island) and sank. Also lost were a second destroyer, a cargo ship, four gunboats, and several smaller units. A seaplane tender and a cargo ship were heavily damaged. U.S. forces lost two fighter planes and one torpedo plane.
The Yorktown and her escort ships turned back immediately after this attack and rejoined the other ships on 5 May to take on fuel again from the remaining tankers. Shortly thereafter, aircraft from the Yorktown shot down a Japanese flying boat. A Japanese submarine sighted a little later, which had probably been guided to the U.S. fleet by this flying boat, turned away again.
The U.S. units took up a position about 1,100 kilometers south of Rabaul and waited for the Japanese main fleet to advance. As reports of a concentration of ships headed for Port Moresby increased, Fletcher ordered a northerly course to attack the Japanese on the morning of 7 May. The tanker Neosho and the destroyer USS Sims were ordered to operate south of the fleet. Another group, Task Force 44, under the command of Rear Admiral John Crace, was to intercept Japanese transports and their escort ships en route to Port Moresby. The force consisted of the heavy cruisers HMAS Australia and USS Chicago, the light cruiser HMAS Hobart, and the destroyers USS Perkins, USS Walke, and USS Farragut. As the ships reached a position 180 km off the southern tip of New Guinea, they were attacked by 27 Japanese aircraft. Only minutes after the Japanese attack ended, U.S. B-17 bombers that had taken off from Australian air bases mistakenly bombed the formation. However, there was little significant damage in either attack.