The Malayan campaign, referred to by Japanese sources as the Malay Operation (馬来作戦, Maree Sakusen), was a military campaign fought by Allied and Axis forces in Malaya from 8 December 1941 – 15 February 1942 during the Second World War. It was dominated by land battles between British Commonwealth army units and the Imperial Japanese Army, with minor skirmishes at the beginning of the campaign between British Commonwealth and Royal Thai Police. The Japanese had air and naval supremacy from the opening days of the campaign. For the British, Indian, Australian, and Malayan forces defending the colony, the campaign was a total disaster.

The operation is notable for the Japanese use of bicycle infantry, which supposedly allowed troops to carry more equipment and swiftly move through thick jungle terrain. Royal Engineers, equipped with demolition charges, destroyed over a hundred bridges during the retreat, yet this did little to delay the Japanese. By the time the Japanese had captured Singapore, they had suffered 14,768 casualties; allied losses totaled 130,246, including around 7,500 to 8,000 killed, 11,000+ wounded and 120,000+ missing or captured.

By 1941 the Japanese had been engaged for four years in trying to subjugate China. They were heavily reliant on imported materials for their military forces, particularly oil from the United States. From 1940 to 1941, following the Japanese takeover of French colonies the United States, the United Kingdom, and the Netherlands imposed embargoes on supplying oil and war materials to Japan. The object of the embargoes was to assist the Chinese and encourage the Japanese to halt military action in China. The Japanese considered that pulling out of China would result in a loss of face and decided instead to take military action against US, British and Dutch territories in South East Asia. The Japanese forces for the invasion were assembled in 1941 on Hainan Island and in French Indochina. This troop build-up was noticed by the Allies and, when asked, the Japanese advised that it related to its operations in China.

When the Japanese invaded, they had over 200 tanks, consisting of the Type 95 Ha-Go light tank, Type 97 Chi-Ha and Type 89 I-Go medium tanks and Type 97 Te-Ke tankettes. In support they had nearly 800 aircraft available. Commonwealth troops were equipped with the Lanchester 6x4 Armoured Car, Marmon-Herrington Armoured Car, Universal Carrier and only 23 obsolete Mk VIB light tanks (in the 100th Light Tank Squadron of the Indian Army), none of which were sufficiently armed for armoured warfare. They had just over 250 combat aircraft; half of these would be lost in the first few days of combat.

When the Japanese invaded Malaya it was part of expansive attack all across the Pacific and South East Asia, stretching from Pearl Harbor and islands in the Pacific, the Philippines, Hong Kong, Burma, Singapore, Borneo, and Thailand.

Between the wars, the British military strategy in the Far East was undermined by a lack of attention and funding. In 1937, Major-General William Dobbie, General Officer Commanding (GOC) Malaya (1935–39), looked at Malaya's defences and reported that during the monsoon season, from October to March, landings could be made by an enemy on the east coast and bases could be established in Siam (Thailand). He predicted that landings could be made at Songkhla and Pattani in Siam, and Kota Bharu in Malaya. He recommended large reinforcements to be sent immediately. His predictions turned out to be correct, but his recommendations were ignored. The British government's plans relied primarily on the stationing of a strong fleet at the Singapore Naval Base in the event of any enemy hostility, in order to defend both Britain's Far Eastern possessions and the route to Australia. A strong naval presence was also thought to act as a deterrent against possible aggressors.

By 1940, however, the army commander in Malaya, Lieutenant-General Lionel Bond, conceded that a successful defence of Singapore demanded the defence of the whole peninsula, and that the naval base alone would not be sufficient to deter a Japanese invasion. Military planners concluded that the desired Malayan air force strength would be 300–500 aircraft, but this was never reached because of the higher priorities in the allocation of men and material for Britain and the Middle East.^{[citation needed]}

The defence strategy for Malaya rested on two basic assumptions: first, that there would be sufficient early warning of an attack to allow for reinforcement of British troops, and second, that American help was at hand in case of attack. By late 1941, after Lieutenant-General Arthur E. Percival had taken over as GOC Malaya, it became clear that neither of these assumptions had any real substance. In addition, Churchill and Roosevelt had agreed that in the event of war breaking out in South East Asia, priority would be given to finishing the war in Europe. The east, until that time, would be a secondary priority. Containment was considered the primary strategy in the east.^{[citation needed]}