Operation Mo

• 1. Introduction

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  Operation Mo, the Japanese plan to seize Port Moresby and sever Australia's connection to American support, was not improvised. Admiral Isoroku Yamamoto, the commander-in-chief of the Combined Fleet, lent his authority to the scheme. Its strategic logic stretched back years before the Pacific War began. The goal was unambiguous: isolate Australia and New Zealand from the Allied United States. Port Moresby sat in the Australian Territory of New Guinea, and whoever held it commanded the approaches to the continent. The operation's ambitions extended across thousands of miles of ocean. Japan had transformed those ambitions into a formal campaign plan. How did Japan assemble the forces for an operation of this scale? By then, another question was equally urgent: whether the plan had already been undone before it began.

• 2. Four Years, Four Actions

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  The Japanese Navy General Staff had been examining Operation Mo since 1938. The directive was conceived that year, though planners set no specific date for its execution. The launch depended on earlier successes during the first and second phases of Japan's southern conquest. When the time came, the opening New Guinea operations were assigned to the naval task force of Admiral Chūichi Nagumo. Nagumo was earmarked for the role after completing the Java campaign. Planners also designated Christmas Island, south of Java, as another step in consolidating Japan's southern frontier.

  In April 1942, the Army and Navy General Staffs gave their approval. On the 3rd of May, a Light Task Force would occupy the port of Tulagi, near Guadalcanal in the Solomon Islands. The purpose was to establish a seaplane base for Coral Sea operations. That same force was also assigned to seize Nauru and Banaba, two atolls prized for their phosphate deposits. On the 10th of May, the South Seas Detachment was to disembark at Port Moresby. A second force would simultaneously secure the Louisiade Archipelago for another seaplane base. The Imperial General Headquarters gave the South Seas Detachment a double objective. First, capture Port Moresby in cooperation with the Navy. Second, seize strategic points in eastern New Guinea. An operation designated FS went further still, calling for assaults on New Caledonia, Fiji, and Samoa.

  A naval force departing from Truk was to pass through the Eastern Solomons, advance west, and intercept any Allied interference. Strikes were also planned against the Queensland towns of Coen, Cooktown, and Townsville. These were terminal points in the supply chain between the United States and Australia. The final geographical target was Thursday Island, north of Cape York. Japanese naval intelligence had already flagged the likely presence of an American aircraft carrier in the Coral Sea. Planners positioned intercepting forces on either side of the line between Rennell and Deboyne Islands. The intent was to channel invasion transports through the Jomard Passage directly to Port Moresby. What Japan's planners did not reckon with was the intelligence already assembled against them.

• 3. The Intelligence Breach

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  A Japanese message on the 9th of April 1942 made Japan's intentions plain. It described the "RZP Campaign" against Port Moresby as an invasion, not an air raid. United States Navy codebreaker Rudy Fabian, working at FRUMEL, brought this intelligence directly to General MacArthur. MacArthur was "incredulous." He had expected the target to be New Caledonia, not Port Moresby.

  Fabian did not simply hand over a summary. He walked MacArthur through the JN-25 code-breaking process and showed him the intercepted message from the Japanese commander. The text laid out the operation's objectives: restrict enemy fleet movements and attack the north coast of Australia. On the 23rd of April, Fabian returned with a second briefing. He showed that the Imperial Japanese Navy was amassing a large force at Truk and also intended to occupy Tulagi. A transport ship due to leave the following day for New Caledonia was diverted to Port Moresby instead.

  MacArthur arranged for an Australian reconnaissance aircraft to "discover" the Port Moresby invasion force on the 5th of May. The word "discover" belongs in quotation marks: Allied commanders already knew exactly what was approaching. This staged sighting gave the Allied response a public-facing trigger. The result was the Battle of the Coral Sea.

• 4. Ships and Aircraft

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  Rear Admiral Kiyohide Shima led the Tulagi assault force. His command included a minelayer-cruiser, a seaplane tender, four destroyers, two transports, and smaller support vessels. The Port Moresby occupation force was under Rear Admiral Sadamichi Kajioka. Kajioka's command comprised a light cruiser, six destroyers, minelayers, sea patrol vessels, a seaplane tender, and a further minelayer.

  The 25th Air Fleet, designated the Yokohama Air Corps, was led by Rear Admiral Sadayoshi Yamada. Yamada's bases spanned Rabaul, Lae, Salamaua, Buna, and Deboyne Islands. His unit put 60 Mitsubishi A6M "Zero" fighters into the air, alongside 48 Mitsubishi G3M "Nell" bombers. It also deployed 26 reconnaissance seaplanes of the Aichi E13A "Jake" and Mitsubishi F1M "Pete" types. This air fleet struck Port Moresby on the 5th and the 6th of May. The purpose was to prepare the way for a planned Army-Navy landing on the 7th of May. The minelayer-cruiser and seaplane tender in Shima's Tulagi force were among the vessels lost before the operation concluded.

• 5. Coral Sea Collapse

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  On the 3rd of May, the Tulagi assault force began its landings. The following day, on the 4th of May 1942, troopships bearing the South Seas Detachment set sail southward from Rabaul toward Port Moresby. United States aircraft from the carrier Yorktown attacked the Tulagi assault force that same day and inflicted heavy damage. The occupation of Tulagi, Gavutu, and Tanambogo islands proceeded despite the attack.

  Three days after the Tulagi landings, a naval engagement was clearly developing in the Coral Sea. The Japanese Moresby transports immediately reversed course northward to avoid combat. The Battle of the Coral Sea inflicted significant aircraft losses on Japan's Fourth Fleet. Shōhō was sunk. Shōkaku was damaged. Air groups from both fleet carriers suffered such severe losses that a return to Japan for re-equipping and retraining was unavoidable. Zuikaku, though it came through the battle relatively intact, lost enough of its air group to require the same withdrawal.

  Japan abandoned its plan to put the South Seas Detachment ashore at Port Moresby from the sea. The Japanese Army's next approach to Port Moresby would not involve the sea.

• 6. The Kokoda Turn

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  On the 11th of July, Japan's High Command ordered the suspension of Operation FS. The planned assaults further into the Pacific were called off. The decisive blow had come at the Battle of Midway, where Japan's remaining carrier strength was destroyed. Without those carriers, the naval reach that Operation Mo required was gone.

  The Owen Stanley Range stood between Japan's positions in eastern New Guinea and Port Moresby. The Japanese Army attempted an overland march across the range via the Kokoda Track. That campaign ended in Japanese withdrawal. The Kokoda Track had been Japan's last path to Port Moresby, and Port Moresby would never fall.