On 3–4 May, Japanese forces invaded and occupied Tulagi. The Japanese aircraft carriers entered the Coral Sea to try to destroy the Allied naval forces.
On 7 May, the carrier forces from the two sides sent planes to attack the ships of the other side. The first day, the U.S. sank the Japanese light carrier "Shōhō", while the Japanese sank a U.S. destroyer. The next day, the Japanese carrier "Shōkaku" was badly damaged, and the U.S. carriers "Lexington" and the "Yorktown" were damaged. Since both sides suffered heavy losses in aircraft and carriers, the two fleets stopped the battle.
The Japanese sank more ships than the U.S. However, the battle was considered a victory for the Allies because the Japanese forces were not able to capture the locations they were hoping to occupy. As well, the Japanese carriers "Shōkaku" and "Zuikaku" were not able to fight in the Battle of Midway, which helped the U.S. to win there. Japan's losses of carriers meant that they could not invade Port Moresby. Two months later, the Allies launched the Guadalcanal Campaign.
On 7 December 1941, using aircraft carriers, the Japanese attacked the U.S. Pacific fleet at Pearl Harbor, Hawaii. The attack destroyed most of the U.S. Pacific Fleet's battleships. It also started a state of war between the two nations. The Japanese wanted to destroy American navy ships, capture land with natural resources, and obtain military bases to defend their empire.
At the same time that they were attacking Pearl Harbor, the Japanese attacked Malaya. This caused the United Kingdom, Australia, and New Zealand to join the United States as Allies in the war against Japan (Australia had joined World War 2 in 1939 when Germany invaded Poland). The goals of the Japanese battles in the war were to remove British and Americans from the Netherlands Indies and the Philippines.
In the first few months of 1942, Japanese forces attacked and captured the Philippines, Thailand, Singapore, the Netherlands East Indies, Wake Island, New Britain, the Gilbert Islands, and Guam. They also destroyed a lot of Allied land, naval, and air forces. Japan planned to use these lands to defend its empire.
Shortly after the war began, Japan's Naval General Staff wanted to invade Northern Australia. The goal was to prevent Australia from being used as a base to threaten Japan's defenses in the South Pacific.
The Imperial Japanese Army (IJA) said it did not have the forces or ships to invade Australia. Vice Admiral Shigeyoshi Inoue, commander of the IJN's 4th Fleet (also called the South Seas Force) had the idea of capturing Tulagi in the southeastern Solomon Islands and Port Moresby in New Guinea.
This would put northern Australia within range of Japanese land-based aircraft. Japan decided to capture New Caledonia, Fiji, and Samoa. This would make it hard for the United States to supply Australia.
In April 1942, the army and navy developed a plan called Operation "MO". The plan was to invade Port Moresby by 10 May. The plan also included capturing Tulagi on 2–3 May. This would give the navy a base for attacks against Allied territories and forces in the South Pacific.
When "MO" was done, the navy planned to do Operation "RY". This was a plan to capture Nauru and Ocean Island for their phosphate deposits on 15 May.
Further attacks against Fiji, Samoa and New Caledonia were planned. There was a damaging air attack by Allied aircraft on Japanese naval forces invading the Lae-Salamaua area in New Guinea in March. Inoue requested carriers to provide airplanes. Inoue was worried about Allied bombers at air bases in Townsville and Cooktown, Australia.
Admiral Isoroku Yamamoto, commander of Japan's Combined Fleet, was planning an attack for June. He wanted to try to destroy the U.S. Navy's carriers. None were damaged in the Pearl Harbor attack.
Unknown to the Japanese, the U.S. Navy had decoded Japanese secret codes. By March 1942, the U.S. was able to figure out up to 15% of the IJN's code. By the end of April the Americans were reading up to 85% of the messages in code.
In March 1942, the U.S. first noticed the "MO" operation in messages. On 13 April, the British decoded an IJN message telling Inoue that the Fifth Carrier Division, consisting of the fleet carriers and , was being sent. The British sent the message to the Americans. They also said that Port Moresby would probably be attacked in the "MO" plan.
Admiral Chester Nimitz, the new commander of Allied forces in the Pacific, and his staff thought the Japanese were planning an attack in early May on Port Moresby. The Allies saw Port Moresby as an important base for attacking the Japanese. Nimitz's staff also thought that the Japanese might attack Allied bases in Samoa and at Suva.
Nimitz sent all four of the Pacific fleet's aircraft carriers to the Coral Sea. By 27 April, Japanese messages helped the allies to know most targets of the "MO" and "RY" plans.
On 29 April, Nimitz sent his four carriers and their supporting warships towards the Coral Sea. Task Force 17 consisted of the carrier , three cruisers and four destroyers. It was supported by two oilers and two destroyers.
Task Force 11 consisted of the carrier with two cruisers. TF 16 included the carriers and the USS Hornet, but they were too far away.
Nimitz put Fletcher in command of Allied naval forces in the South Pacific area until Halsey arrived Halsey was told to command all three task forces once TF 16 arrived in the Coral Sea area (Lundstrom, "Pearl Harbor to Midway", p. 167).
The Japanese thought that all but one of the U.S. Navy's carriers were in the central Pacific. The Japanese did not know the location of the other carrier, but they did not expect an American carrier response to "MO" until the attacks had begun.
During late April, the Japanese submarines and searched the area where landings were planned. The submarines explored Rossel Island and the Deboyne Group area and the route to Port Moresby. They did not see any Allied ships and returned to Rabaul on 23 and 24 April.
The Japanese Port Moresby Invasion Force, commanded by Rear Admiral Kōsō Abe, included 11 transport ships carrying about 5,000 soldiers from the IJA's South Seas Detachment plus 500 more troops.
This included one light cruiser and six destroyers under the command of Rear Admiral Sadamichi Kajioka. Abe's ships departed Rabaul for the trip to Port Moresby on 4 May and were joined by Kajioka's force the next day. The ships, planned to arrive at Port Moresby by 10 May.
The Allied forces at Port Moresby had 5,333 men, but only half of these were infantry and all had poor equipment and little training.
Leading the invasion of Tulagi was the Tulagi Invasion Force. It was commanded by Rear Admiral Kiyohide Shima. It consisted of two minelayers, two destroyers, six minesweepers, two subchasers, and a transport ship carrying about 400 troops. Supporting the Tulagi force was the light carrier , four heavy cruisers, and one destroyer, commanded by Rear Admiral Aritomo Gotō.
There was a separate force commanded by Rear Admiral Kuninori Marumo. It consisted of two light cruisers, the seaplane tender , and three gunboats. Inoue directed "MO" from the cruiser . He arrived on 4 May.
Gotō's force left Truk on 28 April and stayed near New Georgia Island. Marumo's support group left New Ireland to establish a seaplane base on 2 May to support the Tulagi attack. Shima's invasion force left Rabaul on 30 April.
The Carrier Strike Force with carriers "Zuikaku" and "Shōkaku", two heavy cruisers, and six destroyers left from Truk on 1 May. The strike force was commanded by Vice Admiral Takeo Takagi (flag on cruiser ). Rear Admiral Chūichi Hara, on "Zuikaku", commanded the carrier air forces.
The Carrier Strike Force was to enter the Coral Sea south of Guadalcanal. Once in the Coral Sea, the carriers were to provide airplanes for the invasion forces, destroy Allied airplanes at Port Moresby, and destroy any Allied naval forces in the Coral Sea.