Operation Mo

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Operation Mo
Battle of the Coral Sea.jpg
Map showing the movements of the Port Moresby invasion force, and the plan for the force's landing at Port Moresby
PlannedApril 1942
ObjectiveOccupation of Port Moresby
OutcomeAbandoned following the Battle of the Coral Sea

Operation Mo(MO作戦,Mo Sakusen) or the Port Moresby Operation was a 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. The operation was ultimately abandoned.

Empire of Japan Empire in the Asia-Pacific region between 1868–1947

The Empire of Japan was the historical nation-state and great power that existed from the Meiji Restoration in 1868 to the enactment of the 1947 constitution of modern Japan.

Territory of New Guinea Australian administered territory est. 1920

The Territory of New Guinea was an Australian administered territory on the island of New Guinea from 1920 until 1975. In 1949, the Territory and the Territory of Papua were established in an administrative union by the name of the Territory of Papua and New Guinea. That administrative union was renamed as Papua New Guinea in 1971. Notwithstanding that it was part of an administrative union, the Territory of New Guinea at all times retained a distinct legal status and identity until the advent of the Independent State of Papua New Guinea.

World War II 1939–1945, between Axis and Allies

World War II, also known as the Second World War, was a global war that lasted from 1939 to 1945. The vast majority of the world's countries—including all the great powers—eventually formed two opposing military alliances: the Allies and the Axis. A state of total war emerged, directly involving more than 100 million people from more than 30 countries. The major participants threw their entire economic, industrial, and scientific capabilities behind the war effort, blurring the distinction between civilian and military resources. World War II was the deadliest conflict in human history, marked by 70 to 85 million fatalities, most of whom were civilians in the Soviet Union and China. It included massacres, the genocide of the Holocaust, strategic bombing, premeditated death from starvation and disease, and the only use of nuclear weapons in war.



When the Japanese Navy was planning the New Guinea Campaign (air strikes against Lae and Salamaua, disembarkation in Huon Gulf, New Britain (Rabaul), New Ireland (Kavieng), Finch Harbor (also called Finschhafen), and the capture of Morobe and Buna), strategists envisioned those territories as support points to implement the capture of Port Moresby. The implementation of these operations was assigned to the Japanese Naval task force led by Admiral Chūichi Nagumo, after completing the Java campaign. Another important step 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 consolidation of the Southern Seas areas in the Greater East Asia Co-Prosperity Sphere (大東亜共栄圏,Dai-tō-a Kyōeiken).[ citation needed ]

Lae City in Morobe, Papua New Guinea

Lae is the capital of Morobe Province and is the second-largest city in Papua New Guinea. It is located near the delta of the Markham River and at the start of the Highlands Highway, which is the main land transport corridor between the Highlands region and the coast. Lae is the largest cargo port of the country and is the industrial hub of Papua New Guinea. The city is known as the Garden City and home of the University of Technology.

Salamaua Place in Morobe Province, Papua New Guinea

Salamaua was a small town situated on the northeastern coastline of Papua New Guinea, in Salamaua Rural LLG, Morobe province. The settlement was built on a minor isthmus between the coast with mountains on the inland side and a headland. The closest city is Lae, which can be reached only via boat across the gulf.

Huon Gulf gulf in Papua New Guinea

Huon Gulf is a large gulf in eastern Papua New Guinea, at 7.0°S 147.45°E. It is bordered by Huon Peninsula in the north. Both are named after French explorer Jean-Michel Huon de Kermadec. Huon Gulf is a part of the Solomon Sea. Lae, capital of the Morobe Province is located on the northern coast of the gulf.


The Directive of Operation Mo was conceived in 1938, but with no specific time for its execution, pending earlier successes in the southern area during the first and second phases of the conquest.

In April 1942, the operation was organized into four large actions and was approved by the Army and Navy General Staffs:

Tulagi Small island in the Solomon Islands north of Guadalcanal

Tulagi, less commonly known as Tulaghi, is a small island in Solomon Islands, just off the south coast of Ngella Sule. The town of the same name on the island was the capital of the British Solomon Islands Protectorate from 1896 to 1942 and is today the capital of the Central Province. The capital of what is now the state of Solomon Islands moved to Honiara, Guadalcanal, after World War II.

Guadalcanal island of the Solomon Islands chain in the South Pacific Ocean

Guadalcanal is the principal island in Guadalcanal Province of the nation of Solomon Islands, located in the south-western Pacific, northeast of Australia. The island is mainly covered in dense tropical rainforest and has a mountainous interior.

Solomon Islands Country in Oceania

Solomon Islands is a sovereign state consisting of six major islands and over 900 smaller islands in Oceania lying to the east of Papua New Guinea and northwest of Vanuatu and covering a land area of 28,400 square kilometres (11,000 sq mi). The country's capital, Honiara, is located on the island of Guadalcanal. The country takes its name from the Solomon Islands archipelago, which is a collection of Melanesian islands that also includes the North Solomon Islands, but excludes outlying islands, such as Rennell and Bellona, and the Santa Cruz Islands.

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 the American aircraft carrier USS Yorktown in Coral Sea waters during this period.

Deboyne Islands group of islands in the Solomon Sea, Papua New Guinea

The Deboyne Islands are an atoll, composed of a group of reefs and islands in the north of the Louisiade Archipelago, Papua New Guinea.

Aircraft carrier Warship that serves as a seagoing airbase

An aircraft carrier is a warship that serves as a seagoing airbase, equipped with a full-length flight deck and facilities for carrying, arming, deploying, and recovering aircraft. Typically, it is the capital ship of a fleet, as it allows a naval force to project air power worldwide without depending on local bases for staging aircraft operations. Carriers have evolved since their inception in the early twentieth century from wooden vessels used to deploy balloons to nuclear-powered warships that carry numerous fighters, strike aircraft, helicopters, and other types of aircraft. While heavier aircraft such as fixed-wing gunships and bombers have been launched from aircraft carriers, it is currently not possible to land them. By its diplomatic and tactical power, its mobility, its autonomy and the variety of its means, the aircraft carrier is often the centerpiece of modern combat fleets. Tactically or even strategically, it replaced the battleship in the role of flagship of a fleet. One of its great advantages is that, by sailing in international waters, it does not interfere with any territorial sovereignty and thus obviates the need for overflight authorizations from third party countries, reduce the times and transit distances of aircraft and therefore significantly increase the time of availability on the combat zone.

USS <i>Yorktown</i> (CV-5) 1937 Yorktown-class aircraft carrier of the United States Navy

USS Yorktown (CV-5) was an aircraft carrier commissioned in the United States Navy from 1937 until she was sunk at the Battle of Midway in June 1942. Yorktown played an important part in the battle, assisting in the destruction of multiple enemy aircraft carriers, and absorbing the majority of enemy counter attacks. She was named after the Battle of Yorktown in 1781 and the lead ship of the Yorktown class which was designed after lessons learned from operations with the large converted battlecruiser Lexington class and the smaller purpose-built USS Ranger. She was sunk by Japanese submarine I-168 on 6 June 1942 during the Battle of Midway.

A Japanese message on 9 April 1942 indicated that the "RZP Campaign" against Port Moresby was to be an invasion not an air raid and which would isolate Australia from America. USN codebreaker Rudy Fabian at FRUMEL briefed MacArthur who was "incredulous" as he expected the target would be New Caledonia, so Fabian explained the (JN-25) code-breaking process and showed him the Japanese commander’s intercepted message with the objectives to restrict enemy fleet movements and attack the north coast of Australia. So a transport due to leave the next day for New Caledonia was sent to Port Moresby. On 23 April Fabian showed MacArthur that the IJN was amassing a large force at Truk, and planned to occupy Tulagi also. MacArthur got an Australian reconnaissance plane to "discover" the Port Moresby invasion force on 5 May, leading to the Battle of the Coral Sea (see Central Bureau). [1]

Fleet Radio Unit, Melbourne (FRUMEL) was a United States–Australian–British signals intelligence unit, founded in Melbourne, Australia, during World War II. It was one of two major Allied signals intelligence units called Fleet Radio Units in the Pacific theatres, the other being FRUPAC, in Hawaii. FRUMEL was a US Navy organisation, reporting directly to CiCPAC in Hawaii and the Chief of Naval Operations in Washington, and hence to the central cryptographic organization. The separate Central Bureau in Melbourne was attached to MacArthur's Allied South West Pacific Area command headquarters.

Battle of the Coral Sea Major naval battle in the Pacific Theater of World War II

The Battle of the Coral Sea, fought from 4–8 May 1942, was a major naval battle between the Imperial Japanese Navy (IJN) and naval and air forces from the United States and Australia, taking place in the Pacific Theatre of World War II. The battle is historically significant as the first action in which aircraft carriers engaged each other, as well as the first in which the opposing ships neither sighted nor fired directly upon one another.

Order of battle

The Tulagi assault force, led by Rear Admiral Kiyohide Shima, was composed of the following units:

Kiyohide Shima Japanese admiral

Kiyohide Shima was an admiral in the Imperial Japanese Navy during World War II.

The Port Moresby occupation force, led by Rear Admiral Sadamichi Kajioka, was composed of the following units:

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

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

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 Islands, composed of 60 Mitsubishi A6M "Zero" fighters, 48 Mitsubishi G3M "Nell" and 26 Aichi E13A "Jake" and Mitsubishi F1M "Pete" reconnaissance seaplanes. This unit bombed Port Moresby on 5–6 May, ahead of the Japanese Army-Navy landing on 7 May.


The Tulagi assault force began their landings on Tulagi on 3 May. On 4 May 1942, troopships bearing the South Seas Detachment set sail southward from Rabaul for Port Moresby. This same day US aircraft from Yorktown attacked the Tulagi assault force, inflicting heavy damage, but were unsuccessful in preventing the occupation of Tulagi, Gavutu, and Tanambogo islands.

Three days later, as a naval engagement appeared to be brewing in the Coral Sea, the Japanese Moresby transports immediately veered back to the north, in order to avoid combat. The resulting Battle of the Coral Sea inflicted significant aircraft losses on the Fourth Fleet, Shōhō was sunk, and Shōkaku was damaged. Air groups from the two carriers, including the relatively undamaged Zuikaku, suffered such sizable losses, it was necessary they return to Japan to re-equip and train.

The Japanese abandoned their plans to land the South Seas Detachment directly at Port Moresby from the sea. The Japanese Army was making new preparations for combat when, on 11 July, High Command ordered the suspension of Operation FS the projected actions against New Caledonia, Fiji, and Samoa, after the remaining Japanese carrier strength was destroyed at the Midway.

These battles prevented the Japanese landings against Port Moresby. Instead the Japanese army commenced an ultimately unsuccessful campaign to take Port Moresby with an overland approach across the Owen Stanley Range via the Kokoda Track.

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Japanese cruiser <i>Myōkō</i>

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Tone was the lead ship in the two-vessel Tone class of heavy cruisers in the Imperial Japanese Navy. The ship was named after the Tone River, in the Kantō region of Japan and was completed on 20 November 1938 at Mitsubishi's Nagasaki shipyards. Tone was designed for long-range scouting missions and had a large seaplane capacity. She was extensively employed during World War II usually providing scouting services to their aircraft carrier task forces. She almost always operated in this capacity in conjunction with her sister ship Chikuma.

Battle of the Coral Sea order of battle

This is an order of battle for the Battle of the Coral Sea. The battle, fought during 4–8 May 1942, was a major naval battle in the Pacific Theater of World War II between the Imperial Japanese Navy and Allied naval and air forces from the United States (U.S.) and Australia.

South Seas Detachment

The South Seas Detachment of the Imperial Japanese Army was a brigade-size force formed in 1941 to be the army unit used in the Japanese seizure of the South Pacific island groups of Wake, Guam and the Gilberts. As part of the South Seas Force, it fell under Imperial Japanese Navy command and control. It was drawn from the 55th Division and was commanded by Major General Tomitarō Horii. It consisted of the following units:

Japanese cruiser <i>Tenryū</i> lead ship of Tenryū-class of light cruisers

Tenryū (天龍) was the lead ship in the two-ship Tenryū class of light cruisers of the Imperial Japanese Navy. Tenryū was named after the Tenryū River in Nagano and Shizuoka prefectures.

Japanese cruiser <i>Tatsuta</i> (1918) Tenryū class of light cruiser

Tatsuta (龍田) was the second ship in the two ship Tenryū class of light cruisers in the Imperial Japanese Navy (IJN). She was named after the Tatsuta River in Nara Prefecture, Japan.

New Guinea campaign part of the Pacific Theater of World War II

The New Guinea campaign of the Pacific War lasted from January 1942 until the end of the war in August 1945. During the initial phase in early 1942, the Empire of Japan invaded the Australian-administered territories of the New Guinea Mandate and Papua and overran western New Guinea, which was a part of the Netherlands East Indies. During the second phase, lasting from late 1942 until the Japanese surrender, the Allies—consisting primarily of Australian and US forces—cleared the Japanese first from Papua, then the Mandate and finally from the Dutch colony.

Japanese cruiser <i>Isuzu</i> Nagara-class light cruiser

Isuzu (五十鈴) was the second of six vessels in the Nagara class of light cruisers, and like other vessels of her class, she was intended for use as the flagship of a destroyer flotilla. She was named after the Isuzu River, near Ise Shrine in the Chūbu region of Japan. She saw action during World War II in the Battle of Hong Kong and in the Solomon Islands campaign, and the Battle of Leyte Gulf before being sunk by American submarines in the Netherlands East Indies in April 1945.

Japanese cruiser <i>Kashima</i>

Kashima was the second vessel completed of the three light cruisers in the Katori class, which served with the Imperial Japanese Navy during World War II. The ship was named after the noted Shinto shrine Kashima Jingu in Ibaraki prefecture, Japan.

Japanese cruiser <i>Kako</i> Japanese cruiser

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Invasion of Tulagi (May 1942)

The invasion of Tulagi, on 3–4 May 1942, was part of Operation Mo, the Empire of Japan's strategy in the South Pacific and South West Pacific Area in 1942. The plan called for Imperial Japanese Navy troops to capture Tulagi and nearby islands in the British Solomon Islands Protectorate. The occupation of Tulagi by the Japanese was intended to cover the flank of and provide reconnaissance support for Japanese forces that were advancing on Port Moresby in New Guinea, provide greater defensive depth for the major Japanese base at Rabaul, and serve as a base for Japanese forces to threaten and interdict the supply and communication routes between the United States and Australia and New Zealand.

A series of raids on Deboyne were conducted by Allied forces against the Imperial Japanese Navy seaplane base in the Deboyne Islands of the Louisiade Archipelago between 9–11 May 1942. The seaplane base had been set up prior to the Battle of Coral Sea and became untenable and was abandoned by the Japanese, due to proximity to Allied airfields at Port Moresby and the failure of Mo Sakusen.

William Bowen Ault was a commander in the United States Navy during World War II and a posthumous recipient of the Navy Cross.

Japanese minelayer <i>Okinoshima</i>

Okinoshima (沖島) was a large minelayer of the Imperial Japanese Navy, which was in service during the early stages of World War II. She was named after the Okinoshima Island in the Sea of Japan and the earlier Japanese battleship Okinoshima. She was the largest purpose-built minelayer of the Imperial Japanese Navy and the first Japanese minelayer to be equipped with a reconnaissance seaplane.

Japanese minelayer <i>Tsugaru</i>

Tsugaru (津軽) was a large minelayer of the Imperial Japanese Navy that was in service during the early stages of World War II. She was named after the earlier Japanese cruiser Tsugaru, which in turn was named after Tsugaru Peninsula in northwest Aomori Prefecture of Japan. She was commissioned immediately before the start of World War II, and sunk by a US submarine in June 1944.

Japanese destroyer <i>Mochizuki</i> (1927)

The Japanese destroyer Mochizuki was one of twelve Mutsuki-class destroyers, built for the Imperial Japanese Navy (IJN) during the 1920s. During the Pacific War, she participated in the Battle of Wake Island in December 1941 and the occupations of New Guinea and the Solomon Islands in early 1942.

Operation RY

Operation RY was the Imperial Japanese plan to invade and occupy Nauru and Ocean islands in the south Pacific during the Pacific conflict of World War II. The operation was originally set to be executed in May 1942 immediately following Operation MO and before Operation MI, which resulted in the Battle of Midway. The primary reason for the operation was to exploit the islands' supplies of phosphate. After a postponement due to interference by enemy forces, the operation was completed in August 1942.

Invasion of Salamaua–Lae

The Invasion of Salamaua–Lae, called Operation SR by the Japanese, was an operation by Imperial Japanese forces to occupy the Salamaua–Lae area in the Territory of New Guinea during the Pacific campaign of World War II. The Japanese invaded and occupied the location in order to construct an airfield and establish a base to cover and support the advance of Japanese forces into the eastern New Guinea and Coral Sea areas. The small Australian garrison in the area withdrew as the Japanese landed and did not contest the invasion.


  1. Dufty 2017, pp. 123-128.