In early 1942, the Japanese Empire had a string of unparalleled successes. It had invaded and captured Wake Island, the Philippines, Burma, Malaya, and the Dutch East Indies. But that would all change in the Battle of the Coral Sea in May.

The battle marked the first time in naval history that opposing fleets fought without seeing each other and was the first aircraft carrier battle. But more importantly, it also stopped the Japanese advance.

Japan Sets its Sights on Australia

The Japanese were ready to keep pushing southwest and had their eyes set on capturing Port Moresby in New Guinea. This would isolate Australia, from where they expected that the Allies would launch an inevitable counterattack. So, by occupying Papua, Fiji, Samoa, part of the Solomons, Nauru, and Ocean Islands, the Japanese would create a defensible southeastward perimeter. This would also allow them to interrupt the lines of communication between the U.S. and Australia. 

Admiral Nagano and the Naval General Staff wanted to continue their westward expansion towards Ceylon or India. Nevertheless, Admiral Yamamoto and the Combined Fleet believed that the only way for Japan to defend her interests would be the destruction of the American aircraft carriers. 

So, Yamamoto suggested that Japan move against Midway and then threaten Hawaii where the American carrier fleet could be dealt a death blow. Then, with their West Coast threatened, the Americans would sue for peace. This would give the Japanese the quick victory they needed before the American industrial might could be brought to bear.  

But the Doolittle Raid changed everything as American land-based B-25 bombers, launched off American aircraft carriers, bombed Japan and Tokyo itself. Although the damage was very slight, the fact that American bombers could attack Tokyo was shocking to Japanese leaders.

Dauntless dive bombers of the USS Yorktown prepare for operations against the Japanese during the Battle of the Coral Sea.

The Americans Break the Japanese Naval Code

Nagano ordered Yamamoto to proceed with his Midway plan preceded by a diversionary move 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. As a result, during the Battle of the Coral Sea, the Japanese would be facing a force of equal strength for the first time. 

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