Operation MO - May, 1942

A central front in the war with Japan took place in the electromagnetic spectrum. That front was getting hotter.

The tactical surprise of Japan's attack on Pearl Harbor was made possible by Japanese communications security. Communications were sent and received over land lines and undersea cables. Very few communications were sent by radio, using Japan's naval code, known by western cryptographers as JN-25. On top of that, Japan changed the code on December 4, 1941. Prior to December 4, cryptographers had recovered about 10% of the code. After December 4, they had to start from scratch.

That all changed after December 7, as Japanese naval forces operated throughout East Asia, Southeast Asia, the Indian Ocean, and along the expanding periphery of the "Greater East Asia Co-Prosperity Sphere," as they called their empire. They had to communicate by radio.

More radio communications - more intercepts. Allied cryptographers had much more to work with. By April, they could read more than 50% of the intercepts. It was hard work. They combined the disciplines of cryptanalysis, traffic analysis, and other sources of electronic intelligence. They key punched the intercepts into IBM cards and fed them through IBM machines.

By late April they knew Japan was planning Operation MO - an expansion into the Solomons and an amphibious assault on the south coast of New Guinea to take the area around Port Moresby. It would give them airfields from which they could threaten Australia and interdict the sea routes from the US.

On May 3, 1942, Japanese forces invaded and occupied Tulagi, in the Australian protectorate of the Solomons. It would become a seaplane base.

Admiral Nimitz sent two carrier task forces, centered around the carriers Lexington and Yorktown in the direction of New Guinea. The only other US carriers in the Pacific, Enterprise and Hornet, had just returned to Pearl Harbor from the Doolittle raid on Japan. They got ready to join Lexington and Yorktown.

Japan assigned two carriers, Shokaku and Zuikaku, who had taken part in the attack on Pearl Harbor, as well as a smaller carrier, Shoho.

Cryptanalysis indicated the Japanese invasion of Port Moresby was planned for May 10.