|Battle of the Coral Sea|
|Carrier battle, first day|
|Carrier battle, second day|
|Recovery, reassessment and retreat|
On April 5, the Americans intercepted an IJN message directing a carrier and other large warships to proceed to Inoue's area of operations. On April 13 the British deciphered an IJN message informing Inoue that the Fifth Carrier Division, consisting of the fleet carriers Shōkaku and Zuikaku, was enroute to his command from Formosa via the main IJN base at Truk. The British passed the message to the Americans. Admiral Chester Nimitz, the new commander of Allied forces in the Pacific, and his staff discussed the deciphered messages and concluded that the Japanese were likely initiating a major operation in the Southwest Pacific with Port Moresby as the probable target. The Allies regarded Port Moresby as a key base for a planned counteroffensive, under Douglas MacArthur, against Japanese forces in the southwest Pacific area. Nimitz, after consultation with Admiral Ernest King, Commander in Chief of the United States Fleet, decided to contest the Japanese move against Port Moresby by sending all four of the Pacific fleet's aircraft carriers to the Coral Sea. By April 27, further signals intelligence confirmed most of the details and targets of the Mo and Ry plans.
Leading the invasion of Tulagi was the Tulagi Invasion Force, commanded by Rear Admiral Kiyohide Shima, consisting of two minelayers, two destroyers, six minesweepers, two subchasers, and a transport ship carrying about 400 troops from the 3rd Kure SNLF. Supporting the Tulagi force was the Covering Group with the light carrier Shōhō, four heavy cruisers, and one destroyer, commanded by Rear Admiral Aritomo Gotō. A separate Cover Force (sometimes referred to as the Support Group), commanded by Rear Admiral Kuninori Marumo consisted of two light cruisers, the seaplane tender Kamikawa Maru, and three gunboats joined the Covering Group in providing distant cover for the Tulagi invasion. Once Tulagi was secured on May 3 or 4, the Covering Group and Cover Force were to reposition to help cover the Port Moresby invasion. Inoue directed the Mo operation from the cruiser Kashima, anchored in Simpson Harbour at Rabaul.
Gotō's force left Truk on April 28, pausing briefly at Buka in northern Bougainville on May 1, and took station northwest of New Georgia Island. Marumo's support group sortied from Rabaul early on April 28, helped set-up a seaplane base near the Shortland Islands, then headed for Rekata Bay, Santa Isabel Island to establish another seaplane base on May 2 to support the Tulagi assault. Shima's invasion force departed Rabaul on April 30.
To provide air cover for the Port Moresby invasion, the Carrier Strike Force with carriers Zuikaku and Shōkaku, two heavy cruisers, and six destroyers sortied from Truk on May 1. The strike force was commanded by Vice Admiral Takeo Takagi (flag on cruiser Myōkō) with Rear Admiral Chōichi Hara, on Zuikaku, in tactical command of the carrier air forces. Enroute to the Coral Sea, Takagi's carriers were to deliver nine Zero fighter aircraft to Rabaul. Bad weather, however, during two attempts to make the delivery on May 2 and 3 forced the aircraft to return to the carriers, stationed 240 miles from Rabaul, and one of the Zeros was forced to ditch in the ocean. In order to try to keep to the Mo timetable, Takagi was forced to abandon the delivery mission after the second attempt and directed his force towards the Solomon Islands.
To provide advance warning of the approach of any Allied naval forces, the Japanese had sent four submarines to form a scouting line in the ocean around New Caledonia and the New Hebrides. Fletcher's forces, however, had already passed into the Coral Sea area before the submarines took station, and the Japanese were therefore unaware of their presence. One of the submarines was attacked by Yorktown aircraft on May 2, receiving no damage, but apparently did not realize that it had been attacked by carrier aircraft.
On the morning of May 1, TF17 and TF11 united 250 miles northwest of New Caledonia. Fletcher immediately sent TF11 south to refuel from the oiler Tippecanoe while TF17 refueled from the oiler Neosho. TF17 completed refueling the next day, but TF11 reported that they would not be finished fueling until May 4. Fletcher elected to take TF17 north towards the Louisiades and ordered TF11 to meet Task Force 44 (TF44), which was en route from Sydney and Noumea, on May 4 once refueling was complete. TF44 was a joint Australia-US warship force, commanded by Australian Rear Admiral John Crace, made up of the cruisers HMAS Australia, HMAS Hobart, and USS Chicago, and three destroyers.