 In December 1941 , Japan attacked Pearl Harbor and the east coast of Malaya , causing the Pacific War to begin in earnest . Both attacks occurred at the same time , but due to the international dateline , the Honolulu attack is dated December 7 while the Kota Bharu attack is dated December 8 . One of Japan 's objectives was to capture Southeast Asia and secure the rich supply of natural resources to feed its military and industry needs . Singapore , the main Allied base in the region , was an obvious military target because of its flourishing trade and wealth . The British military commanders in Singapore had believed that the Japanese attack would come by sea from the south , since the dense Malayan jungle in the north would serve as a natural barrier against invasion . Although they had drawn up a plan for dealing with an attack on northern Malaya , preparations were never completed . The military was confident that " Fortress Singapore " would withstand any Japanese attack and this confidence was further reinforced by the arrival of Force Z , a squadron of British warships dispatched to the defense of Singapore , including the battleship HMS Prince of Wales , and cruiser HMS Repulse . The squadron was to have been accompanied by a third capital ship , the aircraft carrier HMS Indomitable , but it ran aground en route , leaving the squadron without air cover . 
