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.
On 8 December 1941, Japanese forces landed at Kota Bharu in northern Malaya. Just two days after the start of the invasion of Malaya, Prince of Wales and Repulse were sunk 50 miles off the coast of Kuantan in Pahang, by a force of Japanese bombers and torpedo bomber aircraft, in the worst British naval defeat of World War II. Allied air support did not arrive in time to protect the two capital ships.[36] After this incident, Singapore and Malaya suffered daily air raids, including those targeting civilian structures such as hospitals or shophouses with casualties ranging from the tens to the hundreds each time.
The Japanese army advanced swiftly southward through the Malay Peninsula, crushing or bypassing Allied resistance.[37] The Allied forces did not have tanks, which they considered unsuitable in the tropical rainforest, and their infantry proved powerless against the Japanese light tanks. As their resistance failed against the Japanese advance, the Allied forces were forced to retreat southwards towards Singapore. By 31 January 1942, a mere 55 days after the start of the invasion, the Japanese had conquered the entire Malay Peninsula and were poised to attack Singapore.[38]