The fall of Singapore on 15 February 1942 broke the United Kingdom's eastern defensive perimeter of the Bay of Bengal; and the Japanese occupation of the Andaman Islands on 23 March gave Japan control of the Andaman Sea enabling ships to resupply Japanese troops in the Burma Campaign for control of India.

[7] Japanese intelligence on the composition of the British Eastern Fleet in the Indian Ocean[8] was reasonably accurate,[6] while overestimating the air strength on Ceylon.

[6] The 19 March operational order vaguely advised that a "considerable" portion of British naval and air forces in the Indian Ocean were "deployed in Ceylon area".

[12] The tonnage and number of ships sunk by Malay Force, are comparable to that of the 3-month long Operation Berlin raid conducted by two battleships of the Kriegsmarine from January to March 1941.

[15] The reinforcement of the British Eastern Fleet depended on transfers from Britain and the Mediterranean, a reflection of active warzones and the demands on the Royal Navy's (RN) resources.

In late-December 1941, a reassessment of the threat posed by Japan envisioned transferring the majority of the RN's heavy units to the Eastern Fleet.

Matters were made urgent by the crippling of the United States Pacific Fleet's battle line at Pearl Harbor, which exposed the weak forces in Malaya to attack.

[16] The Eastern Fleet that Vice Admiral Sir James Somerville assumed command of in March 1942 was smaller than what had been envisioned in December 1941.

[5] Somerville planned to evade the Japanese during the day and close to launch torpedo strikes with radar-equipped Fairey Albacore bombers during the night.

[10] However, the plan was based on information provided by the Far East Combined Bureau (FECB), which identified only two carriers in the Japanese force.

[7] Somerville sailed on 30 March in expectation of an attack on 1 April, and deployed his fleet in a patrol area 100 miles (160 km) south of Ceylon.

[25] Ceylon air defences and forces went on alert,[26] with land-based aerial reconnaissance concentrating on the southeast, where the Japanese were expected to approach to launch strikes at Colombo and Trincomalee.

[26] At about 16:00 on 4 April, PBY Catalina flying boat (AJ155/QL-A) from the Royal Canadian Air Force's (RCAF) 413 Squadron flown by Squadron Leader Leonard Birchall spotted Nagumo's fleet 360 mi (310 nmi; 580 km) south-east of Ceylon[24][25] on a course that would have entered Somerville's previous patrol area from the south.

[24][28] On the morning of 5 April, six Swordfish from 788 Naval Air Squadron (788 NAS) began relocating from China Bay, near Trincomalee, to Colombo, in preparation for a strike on the Japanese fleet.

[8] At dawn, Japanese aerial reconnaissance aircraft flew off to the south-west and north-west; they would fly out to a maximum of 200 miles (320 km) over the next few hours.

Radar-based fighter direction would allow Force A to avoid surprise attack by neutralizing shadowing Japanese aircraft.

The rearming encountered delays, and the strike was carried out by Carrier Division 2 instead; Soryu and Hiryu began launching dive bombers at 11:45.

[41] Carrier Division 2 rejoined the main body's track at 18:00,[42] and caught up at 22:00 180 miles (290 km) due east of Force A.

[30] The Japanese suspected the presence of British carriers, and on the morning of 6 April they launched a much denser[8] westward[43] air search, but found nothing.

[50] Four bombers were shot down over the carriers by CAP A6M2 Zeroes (two of which were claimed by Kaname Harada), and another by Japanese aircraft returning from the strike on Hermes.

[53] The raid demonstrated that the RAF was too weak to defend Ceylon and the naval anchorages,[3] and that the navy was ill-prepared to meet a Japanese carrier force.

British intelligence detected the movement of the Japanese carrier force eastward in mid-April, and their deployment in the Pacific in mid-May.

[58] After the Battle of Midway in June, it was realized that there was no longer the threat of major Japanese naval activity in the Indian Ocean.

The Japanese aircraft carriers required maintenance and replenishment after months of intensive operations,[60] and there was already difficulty in maintaining the strength of frontline air units.

[63] Notably, a submarine group patrolling off East Africa attacked the harbour at Diego-Suarez, Madagascar, while the Allies were capturing the island.

[65] Nagumo's leadership has been characterized by Andrew Boyd as rigid and unimaginative, and contributed to the escape of the British Eastern Fleet.

The manoeuvring of his fleet was mainly to facilitate strikes on Colombo and Trincomalee; the possibility that the enemy might be at sea was apparently not seriously considered.

[10] The failure of the Japanese fleet to appear on 1–2 April led Somerville to mistakenly believe that the entirety, rather than a part, of Allied intelligence concerning Operation C was flawed.

As a result he detached Cornwall, Dorsetshire, and Hermes, which were subsequently lost after being sent into areas overflown by Japanese aerial reconnaissance.

Even then, it required experienced air crews to find their targets at night, using radar with a range of just 20 miles (32 km) and new tactics.