On 9 April 1942 the Japanese attacked the harbor at Trincomalee at 07:00. The British again had warning of the attack, and Hermes and her escorts had left the night before. They were returning to port when they were discovered at 08:55. Hermes had no aircraft on board, and so was defenceless when 70 bombers attacked her at 10:35 off Batticaloa. Hit 40 times, Hermes sank with the loss of 307 men. Vampire and the corvette Hollyhock were also sunk.
The hospital ship Vita later picked up 590 survivors. The RAF lost at least eight Hawker Hurricanes and the Fleet Air Arm one Fairey Fulmar. The Japanese lost five bombers and six fighters, one in a suicide attack on the Trincomalee fuel tanks.
The sortie demonstrated Japanese superiority in carrier operations, and exposed the unprofessional manner in which the RAF was run in the East, but it did not destroy British naval power in the Indian Ocean. It is arguable that, by making full use of signal intercepts, decryption, reconnaissance and superior radar, Somerville was able to save his fast carriers Indomitable and Formidable to fight another day. However, it might equally be said that the blunders made by the Royal Navy meant that the main fleet from Addu was not able to make contact with Nagumo's force as it intended.
An invasion was feared by the British, who interpreted the Japanese failure to do so as due to heavy losses over Ceylon - and hence led to claims of a British victory. However, in reality the Japanese did not have the men, shipping or land based air power to spare for an invasion and occupation and were not even in a position to make a temporary occupation as a raid. The island did not face a real threat of invasion at any point during the war.
The island of Ceylon (Sri Lanka), was strategically important, since it commanded the Indian Ocean. Thus it controlled access to India, also the vital Allied shipping routes to the Middle East and the oilfields of the Persian Gulf. Ceylon held most of the...