With the fall of the Andaman and Nicobar Islands the narrow sea lanes to Singapore and Malaysia were severed. It was obvious an attack on Ceylon was imminent.
Code breakers sifting Japanese radio signals, stated a Japanese force of two or more carriers, battleships of the Kongo class, several eight-inch cruisers, two six-inch cruisers and accompanying destroyers was believed to already at sea with an impending strike on April 1 against Colombo.
Germany was agitating for a fully-fledged invasion of Ceylon. Japan, however, had other priorities.
British intelligence was aware of Hitler’s ambitions. But not of Japan’s reticence.
For Japan, it was a matter of neutralising one last remaining threat: The British Eastern Fleet.
Admiral Kondo's two-pronged advance into the Indian Ocean was now underway. His objective was to bring the Eastern Fleet to battle, and to destroy it. To achieve this, Admiral Nagumo’s elite, and now veteran, carrier strike force was available.
Admiral Nagumo had his flag on the cruiser Chokai. Along with Carrier Division 4, the aircraft carriers Hiyō, Junyō, Shokaku and Zuikaku. Cruiser division 7 , the heavy cruisers Kumano, Suzuya, Mikuma, Mogami and the destroyers Fubuki, Hatsuyuki, Shirayuki, Suzuva and Murakami from the destroyer division 11.
Expected time of arrival: March 31. As a full moon was forecast for April 1, Curteis, Somerville and their advisers were convinced the attack would be launched before dawn that day.
With careful timing, the Eastern Fleet could strike when the Japanese were most vulnerable: At night, when the carriers were preparing to launch a ground attack.
Fearing another costly siege, this time at Rangoon General William Slim, and a careful look over his shoulder at Gandi, ordered an evacuation of General Alexander and all British forces by sea.
Mountbatten's amphibious fleet, Venerable, Hercules, Ocean, Princess Beatrix, Queen Emma, Misoa and Tasajera escorted by the light carriers, Persues, Pioneer and Powerful the heavy cruiser Devonshire, FFS Le Triomphantl, USS Indianapolis, USS Richmond, USS Marblehead, the Hospital ship RFA Empire Clyde and a dozen destroyers sailed on the 28th.
Somerville put to sea on March 30th, with the fleet carriers Invincible, Formidable and Fortitude; the battleships Howe and Duke of York; cruisers Curacoa, Cornwall, Emerald, Enterprise, Caledon, Dragon and eight destroyers Ivanhoe, Inglefield, Imogen, Nestor, Panther, Paladin, Hotspur and Express.
Everything hinged on the success – or otherwise – of reconnaissance. For both navies.
At 18:09 on the 31st, An allied submarine sighted the Japanese ships but lost contact when darkness fell.