The Japanese planned to capture New Guinea, the big and valuable Allied port on Papua’s southern coast called ‘Port Moresby, and to Australia’s northeast, the island of Tulagi along the Solomon chain. By doing this, the Japanese could build airfields on Tulagi to control allied shipping lanes between America and Australia, and enlarge the Japanese empire defensive ring.
The defence of Port Moresby was critical to victory in the South Pacific and to the defence of Australia. Had Port Moresby fallen, it would have left northern Australia more vulnerable to attack.
Singapore and Rabaul had already fallen, and the Japanese troops were getting much closer. Over the period of a year or more, Darwin and northern parts of Australia experienced periodic bombings from the Japanese.
Code-breakers were the biggest reason for the battle. American code-breakers had begun to decipher and intercept the Japanese code and signals. It pointed to a large Japanese naval movement in that area as they prepared to invade more islands and move closer to Australia. The U.S. shifted two of their carriers to that area and eventually the battle came about. The Japanese were not expecting that kind of opposition and their losses during the battle had a direct impact on the battle of Midway.
One of the most significant effects of the Coral Sea was the loss of aircraft carriers, Shokaku and Zuikaku to Japanese Naval Marshal General Yamamoto for his planned showdown with the American carriers at Midway. The Japanese believed that they had sunk two carriers in the Coral Sea, but this still left at least two more US Navy carriers, Enterprise and Hornet, which could help defend Midway. In fact, the Americans would have three carriers to oppose Yamamoto at Midway, because Yorktown remained operational despite the damage from Coral Sea, and the US Navy was able to fix it up sufficiently to enable it participate in the battle.
In the longer term, the Allies gained...