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'Operation Mo' (''Mo Sakusen'') or the 'Port Moresby Operation' was the name of the Japanese plan to take control of the Australian territory of New Guinea during World War II as well as other locations in the South Pacific with the goal of isolating Australia and New Zealand from their ally the United States. The plan was developed by the Imperial Japanese Navy and supported by Admiral Isoroku Yamamoto, the commander-in-chief of the Combined Fleet

Contents

Japanese Navy interest in Operation Mo

Strategic lines in Operation Mo

Japanese countermeasures against Allied response

Proposed Japanese forces

Actual development of Operation Mo

Japanese Navy interest in Operation Mo

During the Japanese Navy's planning of their New Guinea Campaign (air strikes against Lae and Salamaua, disembarkation in Huon Gulf, New Britain (Rabaul), New Ireland (Kavieng), Finch Harbor, and the capture of Morobe and Buna), it envisioned those territories as support points to implement the capture of Port Moresby and a later invasion of Australia. The implementation of these operations was assigned to the Japanese Naval task force led by Admiral Chuichi Nagumo, after finalizing the Java campaign. Another important step in reaching this objective was the occupation of Christmas Island to the south of Java.
The Japanese Navy General Staff had been considering Operation Mo since 1938, as a step in the way to consolidate the Southern Seas areas under the Japanese sphere of influence. A successful occupation of Port Moresby would ease invasion landings of the Australian provinces of Western Australia, Northern Territory and Queensland which according to Japanese navy strategists Port Moresby would serve as an ideal springboard in Allied counteroffensives.

Strategic lines in Operation Mo

The Directive of Operation Mo was conceived in 1938, but with no specific time for its execution, pending the next successes in the southern area during the first and second phases in conquest planning.
In April 1941 the operation was organized into four large actions and was approved by the Army and Navy General Staffs:

★ In May 3 the Light Task Force occupied the port of Tulagi, on Guadalcanal in the Solomon Islands, for the establishment of a base of operations and a hydroplane base for operations in the Coral Sea area. The same force was to take Nauru and Ocean Island for their valuable phosphate deposits.

★ The South Seas Detachment was to disembark in Port Moresby during May 7, along with another force taking territory in the Louisiade Archipelago for the installation of another hydroplane base.

★ Another objective of the South Sea Detachment was the assault of New Caledonia, Fiji, and Samoa. IGHQ assigned a new double objective: the capture and security of Port Moresby, in co-operation with the Navy; and the seizing of strategic points of opportunity in eastern New Guinea.

★ Another important Naval force, departing from Truk, was to pass the Eastern Solomons area, surrounding those islands from the south, finally advancing toward the western area in order to intercept the enemy. Following this, strikes were planned on the coastal cities of Coen, Cooktown and Townsville in Queensland, which were terminal points in the supply line between the United States and Australia. The final object was Thursday Island to the north of Cape York.

★ The Japanese had one Air Naval land-based fleet detached in Rabaul, Lae, Salamaua and Buna. This Air fleet executed the air strikes against Port Moresby during May 5 and May 6, in preparation for the Japanese landing on May 7.

Japanese countermeasures against Allied response

Japanese planners took into account an Allied response to the operation by detaching one task force to the west of parallel between of Rennel and Deboyne Islands and another to the east of same point. These measures would permit a Japanese invasion force to use the Jomard Passage directly to Port Moresby.
Japanese naval intelligence also suspected the presence of American aircraft carrier ''Yorktown'' in Coral Sea waters during this period.

Proposed Japanese forces

The Tulagi assault force was assigned the following units:

★ Minelayer-Cruiser Okinoshima

★ Sea Plane Tender Kiyokawa Maru

★ Destroyers Kikutsuki, Minatsuki, Mochitsuki and Yuziki

★ two Merchant (Transports)

★ smaller support vessels
The Port Moresby occupation force was composed of the following units:

★ Light Cruisers Yubari, Mutzuki, Yagoi, Uzuki, Asanagi, Oite and Yunagi

★ Some minelayers and sea patrol vessels

★ Sea Plane Tender Kamikawa Maru

★ Minelayer Tsugaru
Supporting these operations and intercepting any Allied interference, Rear Admiral Aritomo Goto commanded:

★ Light Carrier Shōhō

★ Heavy Cruisers Aoba, Kiunagasa, Kako and Furutaka

★ Light Cruisers Tenryū, Tatsuta

★ Destroyer Sazanami
During the course of operation, Yamamoto sent the following heavy support force from Truk, led by Read Admiral Chuichi Hara:

★ Heavy Carriers Shōkaku and Zuikaku

★ Heavy Cruisers Myoko and Haguro

★ Destroyers Asashio, Arashio, Arare, Kagero,Shiranuhi and Kasumi

★ Amongst other auxiliar vessels
Supporting this force was the 25th Air Fleet, (Yokohama Air Corps) led by Rear Admiral Sadayoshi Yamada, based in Rabaul, Lae,Salamaua, Buna and Deboyne island, composed of 60 Mitsubishi A6M "Zero" fighters, 48 Mitsubishi G3M "Nell" and 26 Aichi E13A "Jake" and Mitsubishi F1M "Pete" Reconnaissance Hidroplanes. This unit was carried out the aerial bombardment of Port Moresby during May 5-May 6, in preparation of the Japanese Army-Navy disembarkation on May 7.

Actual development of Operation Mo

On May 4, 1942, troopships bearing the South Seas Detachment set sail southward from Rabaul for Port Moresby. Three days later, however, a naval engagement appeared to be brewing in the Coral Sea; whereupon the transports immediately veered back to the north, in order to avoid combat. The Battle of the Coral Sea inflicted great losses on the Fourth Fleet.
Plans to land the South Seas Detachment directly at Port Moresby from the sea had to be abandoned. Army troops were making new preparations for combat when, on July 11, High Command ordered the suspension of the projected actions against New Caledonia, Fiji, and Samoa, because the Combined Fleet had failed at Battle of Midway.
These battles prevented the Japanese disembarkation against Port Moresby and thereby averted the immediate danger of an invasion of Australia.