Before the fall of Singapore, the Eastern Fleet's naval base at Singapore (HM Naval Base) was part of the British Far East Command. British defence planning in the area was based on two assumptions. The first was that the United States would remain as an effective ally in the western Pacific Ocean, with a fleet based at Manila, which would be available as a forward base for British warships. Secondly, the technical capabilities and aggression of the Imperial Japanese Navy were underestimated. In these circumstances, with the Japanese fleet engaged by the United States Navy (USN), the Admiralty planned to send four obsolescent "Revenge"-class battleships to Singapore to provide defensive firepower and a British presence. The British assumptions were destroyed on 7 December 1941: the impact of the Japanese attack on Pearl Harbor denied substantial USN support to the British defence of the "Malay barrier" and made impossible the relief of American garrisons in the Philippines. Furthermore, Japanese capabilities exceeded expectations.