In the spring of 1942, things weren’t going well for the Allies, especially for the Americans in the Pacific. After the crippling sneak attack at Pearl Harbor, the Japanese invaded Wake Island, the Philippines, Burma, Malaya, and the Dutch East Indies, and had American and Filipino forces bottled up at Corregidor. 

The Japanese were ready to keep pushing southwest and had their eyes set on capturing Port Moresby in New Guinea. This would isolate Australia and it would open up even more avenues for the Empire of Japan. But Japanese leaders, flush with success, and the ease with which it had been achieved, were undecided of where to move next. 

Admiral Nagano and the Naval General Staff wanted a westward expansion towards Ceylon, India, or towards Australia. Admiral Yamamoto and the Combined Fleet thought the only way for Japan to defend her interests would be the destruction of the American aircraft carriers. He suggested that they move against Midway and then threaten Hawaii where the American carrier fleet could be dealt a death blow. 

But things changed in April when the inconceivable happened. Japan, and Tokyo itself, were bombed by Jimmy Doolittle and Nagano ordered Yamamoto to proceed with his Midway plan with a feint in the Aleutian Islands. However, the planned operation for Port Moresby and Tulagi had progressed too far to be called off. So, the Imperial Fleet would be running concurrent operations. This would overextend the Japanese forces. 

Operation MO, the operation against Port Moresbly, called for the invasion of the Port and then for an advance towards New Caledonia, Fiji, Samoa, and Tulagi. 

The Japanese didn’t expect any large Allied fleet in the area and no carriers other than the American carrier Saratoga. They were confident that they would easily dispatch any Allied fleet that entered the Coral Sea to stop them. They had two powerful aircraft carriers the Shokaku and the Zuikaku and the light carrier Shoho.

Port Moresby wasn’t just important to the Japanese however. Admiral Chester Nimitz, the Commander-in-Chief of the United States Pacific Fleet (CINCPAC), needed it not just for the defense of Australia, but as a springboard for future moves in the southwestern Pacific. 

The Americans had broken the Japanese codes and knew what their intentions were. However, countering them was another story: The Saratoga wasn’t in the Coral Sea but in Puget Sound undergoing repairs. And aircraft carriers Enterprise and Hornet hadn’t returned from the Doolittle Raid in time. So, Nimitz, keenly aware that Douglas MacArthur would control the land-based aircraft, had to send his own ships there. Nimitz dispatched the air groups of the Yorktown and Lexington.