By the summer of 1941, Japan's relations with the Western powers had deteriorated so much that Japanese leaders saw no point in delaying plans for military operations in Southeast Asia and the Pacific. Japan's short-term goal was to secure the necessary supplies to complete its conquest of China by occupying the Southeast Asian territories controlled by France, Britain, the United States, and the Netherlands. Japan's long-term plans called for further expansion south to Australia and north from Manchuria into the Soviet Union.

Japanese air and naval attacks on British and United States bases in Malaya and the Philippines were coordinated with the December 7, 1941, assault on the United States Pacific Fleet Headquarters at Pearl Harbor, Hawaii. Japan's Southern Army, headquartered in Saigon, quickly moved from bases in southern Indochina and Hainan to attack southern Thailand and northern Malaya on December 8 and the Philippines on December 10. The Japanese easily captured British air bases in northern Malaya and soon controlled the air and sea-lanes in the South China Sea as far south as the Strait of Malacca. Naval landings were made on the Thai coast at Singora (present-day Songkhla) and Patani and on the Malayan coast at Kota Baharu. Also on December 10, the Japanese located and destroyed the Prince of Wales and the Repulse, thereby eliminating the only naval threat to their Malaya campaign. The Thai government capitulated to a Japanese ultimatum to allow passage of Japanese troops through Thailand in return for Japanese assurances of respect for Thailand's independence. This agreement enabled the Japanese to establish land lines to supply their forces in Burma and Malaya through Thailand.

The prediction that Japan would conquer the Malay Peninsula before attempting an invasion of Singapore proved to be correct. Lieutenant General Yamashita Tomoyuki was placed in command of the Twenty-fifth Army comprising three of the best Japanese divisions. The Japanese used tactics developed specifically for the operation in northern Malaya. Tanks were deployed in frontal assaults while light infantry forces bypassed British defenses using bicycles or boats, thereby interdicting British efforts to deliver badly needed reinforcements, ammunition, food, and medical supplies. Cut off from their supply bases in southern Malaya and Singapore, demoralized by the effectiveness of Japan's jungle warfare, and with no possibility that additional ground or air units would arrive in time to turn the tide of battle, the British withdrew to Singapore and prepared for the final siege. The Japanese captured Penang on December 18, 1941, and Kuala Lumpur on January 11, 1942. The last British forces reached Singapore on January 27, 1942, and on the same day a 55-meter gap was blown in the causeway linking Singapore and Johore.

In January 1942, London had provided an additional infantry division and delivered the promised Hurricane fighter aircraft, although the latter arrived in crates and without the personnel to assemble them. In the battle for Singapore, 7cf , the British had the larger ground force, with 70,000 Commonwealth forces in Singapore facing 30,000 Japanese. The Japanese controlled the air, however, and intense bombing of military and civilian targets hampered British efforts to establish defensive positions and created chaos in a city whose population had been swollen by more than a million refugees from the Malay Peninsula. Yamashita began the attack on February 8. Units of the Fifth and Eighteenth Japanese Divisions used collapsible boats to cross the Johore Strait, undetected by the British, to Singapore's northwest coast. By February 13, the Japanese controlled all of the island except the heavily populated southeastern sector. General Percival cabled Field Marshall Sir Archibald Wavell, British Supreme Commander in the Far East, informed him that the situation was hopeless, and received London's permission to surrender. On February 15, one week after the first Japanese troops had crossed the Johore Strait and landed in Singapore, Percival surrendered to Yamashita (see fig. 13).

Data as of December 1989