Japan’s astonishing victories in Malaya, the Philippines and the Netherlands East Indies persuaded its commanders to try and extend their conquests to include Port Moresby, Tulagi, New Caledonia, Fiji and Samoa. Controlling these territories would extend Japan’s defensive perimeter, cut the lines of communication between Australia and the United States and render Australia useless as a base for American forces.

Operation Mo, devised by Admiral Isoroku Yamamoto (1884 – 1943), established seven naval tasks forces to achieve these aims. The most crucial of these were an invasion fleet destined for Port Moresby, protected by the light carrier Shōhō, and a main battle fleet based around the carriersZuikaku and Shōkaku, which would operate further south in the Coral Sea, screening the invasion and preventing Allied interference.

The Japanese task forces were despatched in late April 1942 but their plans were known to the Allies. A joint United States Navy (USN) and Royal Australian Navy (RAN) intelligence unit, called Fleet Radio Unit Melbourne (FRUMEL), had broken the Japanese naval codes and Australian Coastwatchers, operating on Southwest Pacific islands, were monitoring Japanese shipping movements.

A United States carrier task force sailed into the Coral Sea to engage the Japanese battle fleet, while the Australian cruisers HMAS Australia and HMAS Hobart and four American vessels were despatched northwest to intercept the Port Moresby invasion force.

The Battle of the Coral Sea which followed was the first naval battle in history where the opposing fleets never saw each another but fought using carrier aircraft alone. The American carrier USS Lexington, the oiler USS Neosho and the destroyer USS Sims were sunk. Another carrier, USS Yorktown was badly damaged, 66 planes were lost and 543 men killed or injured. The Japanese lost the light carrier Shōhō, the destroyer Kikuzuki, three smaller vessels, 77 aircraft and 1,074 men killed and injured.

After Japanese bombers damaged USS-Yorktown, crewmen climb down ropes and ladders to small boats that transferred them to rescue ships including the destroyer at right, on 4 June 1942, in the Pacific Ocean. Later, a salvage crew returned to the abandoned ship and she made progress towards port, a torpedo from a Japanese submarine destroyed and sank the Yorktown. Image: US Navy

This narrow tactical victory for the Japanese would prove a strategic defeat. Japanese losses at Coral Sea forced them to withdraw their invasion forces, preserving Port Moresby and the other territories and halting their southward advance. The Battle of Midway (4 – 7 June 1942), a month later, signalled the beginning of the end for Japan. Japan’s run of victories was over. Initiative had passed to the Allies.

— Neil Sharkey