Operation Mo
The Japanese Navy General Staff first considered Operation Mo in 1938. This early planning phase focused on consolidating control over Southern Seas areas during the late 1930s. Strategists envisioned territories like New Britain and New Ireland as support points for a future capture of Port Moresby. Admiral Isoroku Yamamoto, commander-in-chief of the Combined Fleet, later supported this specific directive. The plan remained dormant until April 1942 when it was organized into four large actions. Army and Navy General Staffs officially approved the operation at that time. No specific execution date existed initially, pending earlier successes in southern conquest phases.
On the 3rd of May, a Light Task Force occupied the port of Tulagi near Guadalcanal in the Solomon Islands. This force established a seaplane base to operate within the Coral Sea area. The same unit also took Nauru and Banaba Ocean Island for their valuable phosphate deposits. A South Seas Detachment planned to disembark in Port Moresby on the 10th of May. Another force intended to occupy territory in the Louisiade Archipelago for an additional seaplane base. Strikes were also planned against coastal cities including Coen, Cooktown, and Townsville in Queensland. These locations served as terminal points in the supply line between the United States and Australia. Thursday Island sat to the north of Cape York as the final object of the campaign.
A Japanese message dated the 9th of April 1942 indicated that the RZP Campaign against Port Moresby was an invasion. USN codebreaker Rudy Fabian at FRUMEL briefed General MacArthur who found this information incredulous. Fabian explained the JN-25 code-breaking process to show him the Japanese commander's intercepted objectives. He demonstrated plans to restrict enemy fleet movements and attack the north coast of Australia. An Australian reconnaissance plane discovered the Port Moresby invasion force on the 5th of May. This discovery led directly to the Battle of the Coral Sea. On the 23rd of April Fabian showed MacArthur that the IJN was amassing a large force at Truk. They planned to occupy Tulagi also, forcing a transport due for New Caledonia to divert instead.
Troopships bearing the South Seas Detachment set sail southward from Rabaul for Port Moresby on the 4th of May 1942. That same day US aircraft from Yorktown attacked the Tulagi assault force inflicting heavy damage. Troops successfully occupied Tulagi, Gavutu, and Tanambogo islands despite the American strike. Three days later as a naval engagement appeared to be brewing in the Coral Sea, Japanese transports veered back north. The resulting Battle of the Coral Sea inflicted significant aircraft losses on the Fourth Fleet. Shoho was sunk while Shokaku suffered damage during the fighting. Air groups including the relatively undamaged Zuikaku returned to Japan to re-equip and train after such sizable losses.
The Japanese abandoned their plans to land the South Seas Detachment directly at Port Moresby from the sea. High Command ordered the suspension of Operation FS on the 11th of July after carrier strength was destroyed at Midway. These battles prevented the Japanese landings against Port Moresby by sea. Instead the Japanese army commenced an ultimately unsuccessful campaign across the Owen Stanley Range via the Kokoda Track. The Army made new preparations for combat when this overland approach began. Strategic points in eastern New Guinea remained unsecured due to these failures. The initial goal to isolate Australia and New Zealand from the Allied United States failed completely.
Common questions
When was Operation Mo first considered by the Japanese Navy General Staff?
The Japanese Navy General Staff first considered Operation Mo in 1938. This early planning phase focused on consolidating control over Southern Seas areas during the late 1930s.
What specific dates did the Japanese forces occupy Tulagi and Nauru during Operation Mo?
On the 3rd of May, a Light Task Force occupied the port of Tulagi near Guadalcanal in the Solomon Islands. The same unit also took Nauru and Banaba Ocean Island for their valuable phosphate deposits.
Which Japanese admirals commanded the assault forces for Port Moresby and Tulagi in Operation Mo?
Rear Admiral Kiyohide Shima led the Tulagi assault force with a Minelayer-cruiser and Seaplane tender. Rear Admiral Sadamichi Kajioka commanded the Port Moresby occupation force with a Light cruiser and multiple destroyers.
How did US codebreakers reveal the details of Operation Mo to General MacArthur?
USN codebreaker Rudy Fabian at FRUMEL briefed General MacArthur who found this information incredulous. Fabian explained the JN-25 code-breaking process to show him the Japanese commander's intercepted objectives.
When did the Battle of the Coral Sea occur relative to the planned landings on Port Moresby?
An Australian reconnaissance plane discovered the Port Moresby invasion force on the 5th of May. This discovery led directly to the Battle of the Coral Sea which prevented the Japanese landings against Port Moresby by sea.