As I see it Nagumo's purpose was to assess the freedom to operate in the Indian Ocean in making the raid on Ceylon. It may be been enough for him to learn that his fleet was located by a Royal Navy fleet at night in mid ocean for a potential night attack and cause him to withdraw having found the answer to his question. Especially as his fleet was removed to the Pacific afterwards where it was needed.
I missed this the other day. There is no need to speculate about the purpose of Nagumo's operations in the IO as we have the actual orders issued.
The IJN orders from Admiral Kondo, commander of the Southern Force, as issued on 14th March 1942 laid them out clearly and read as follows
"Order No. 139
The Southern Force will carry out the third 'mobile operation' according to the following guidelines. Objectives: destroy the enemy forces on the island of Ceylon by raid attack, cover the Burma invasion operation.
1. Taking advantage of the occasion, Kido Butai will depart from Staring Bay, it will pass Java from the south, then Sumatra from the south-west and carry out an unexpected attack on Ceylon island. The planned date of the raid actions against enemy warships (also air force, navy and other military installations) in the designated area is the beginning of April. Return from the operation is to take place through the Malacca Strait and South China Sea.
2. Of all submarines that are proceeding on the war patrol, the majority will head towards Ceylon (line of the Maldives and Chagos Archipelago) and Bombay with patrol and reconnaissance tasks. At the same time, the remaining units will follow them to the vicinity of Colombo and Trincomalee, to conduct a secret reconnaissance (two days before the planned strike) and provide weather information (on the day of the attack).
3.Current tasks of the air forces are to be continued. Together with Kido Butai, they are to conduct reconnaissance of the Cocos Islands and Christmas Island, as well as neutralise the enemy reconnaissance planes of south-western Sumatra and the Bay of Bengal.
4. The current tasks of the Malay Force are to be continued. Malay Force is to carry out manoeuvring operations along the northern end off the Bay of Bengal against such bases as Madras or the western part of Baby Island. These activities are intended to support Kido Butai's movements. Before the first strike of Kido Butai on Ceylon, the Malay Force cannot cross the 85°E meridian.
5. Except for the forces mentioned above, all other units are to continue their tasks.
6. The main force consisting of The heavy cruiser Atago and the 4th Torpedo Squadron (without the 12th Destroyer Division) is to take advantage to move towards the Strait of Malacca and the Andaman Islands."
See Sasaki - Senshi Sosho vol 26 p623 (translation from Japanese sources)
Following on from that on 19th March Nagumo issued the following orders.
"Kido Butai Secret Operational Instruction No 31.
1. Enemy's Movements
After collecting all intelligence information, it is possible to assume that the Royal Navy in the Indian Ocean consists of two battleships, two aircraft carriers and 11 heavy and armoured cruisers. In India (including Ceylon) there are in total about 500 aircraft. It is estimated that the deployment of forces on the island of Ceylon is proportionate and the enemy may transfer some units towards the Bay of Bengal. Near Australia, there are some undefined remains of ABDA force.
2. Tasks of our forces
Attached to the Order of Southern Force No. 139
3. Acting according to Southern Force Order No.139, Kido Butai is to proceed to the Indian Ocean to destroy the navy and air forces of the enemy in this area
4. Organisation of forces" [See below]
5. contained sailing orders starting 26th March, later delayed.
Table provided is difficult to lay out here but comprises Unit / commander / forces / main tasks.
Kido Butai / commander of First Air Fleet (Nagumo) / First Air Fleet (less Kaga and somecescort destroyers) / destroying enemy's fleet and air force
Support Unit / commander of 3rd Battleship Squadron / 3rd Battleship Squadron & 8th Cruiser Squadron / escorting Kido Butai, attacking enemy's naval force.
Patrol Unit / commander of the 1st Torpedo Squadron / 1st Torpedo Squadron, 17th & 18th Destroyer Divisions, Akigumo, 2nd section of 4th Destroyer Division / patrolling, attacking enemy's naval forces
1st Supply Unit / commander of Shinkoku Maru / Shinkoku Maru, Ken'yo Maru, Nihon Maru, Toei Maru, Kokuyo Maru, Kyokuto Maru
2nd Supply Unit / as above / Hiro Maru, Toei Maru No.2, Hoko Maru.
It was assumed that the Eastern Fleet would be based at Colombo and / or Trincomalee. Primary targets were to be aircraft carriers, battleships and cruisers as well as merchant vessels. Dive bombers were also to focus on the destruction of airfields, hangars, and other military facilities.
Weapon loads for the torpedo bombers were intended to be 250kg & 800kg AP or SAP bombs with 0.02 or 0.03 sec delays.
There is a great deal of detail in "The Darkest Hour vol 1 The Japanese Naval Offensive in the Indian Ocean 1942 - The Opening Moves". The author Michal A Piegzik, as someone working in Japan and speaking the language was able to delve into Japanese archives and produce much more detailed information from that side than was previously available in the West. There is also a vol 2.
The author has also written a number of other books drawing on these sources. He has another about actions in the DEI coming out next year.
