|Japanese troops advance through Kuala Lumpur|
Part of the Japanese work in South-East Asia had already been done for them - the fall of France allowed for them to use the colonial territories of Vichy France as a staging area i.e. French Indochina (Vietnam, Cambodia and Laos). From there, it was a short sail across of the Gulf of Thailand to the British colonial possession of Malaya.
Malaya was an area rich in natural resources that the Japanese Empire needed - rubber and tin, as well as serving to put Borneo, Java and Sumatra, with plenty of oil within their reach. Hit by embargoes from the US, the UK and the Netherlands (the latter controlling Indonesia) for their earlier actions in China, the behemoth needed all of this just to sustain its economy. Add to that further "strength-in-depth" against air attack and the attraction was obvious
Just after midnight on 8 December 1941 (in fact seventy minutes before the Pearl Harbor attack commenced), the Japanese invasion force, hidden by bad weather and in one case shooting a Catalina that had spotted them before it could report their position, landed in Thailand and Malaya. British aircraft tried their best to stop them, but to no avail, although the Japanese did take heavy casualties at Kota Bharu.
The words "to no avail" crop up a lot in this narrative. Again, the British were woefully under prepared for jungle warfare - in fact many thought the Malayan jungle an impenetrable barrier. The Japanese weren't much better, but at least they had a good idea. Bicycles. They could carry far more than a regular foot soldier could and so the Imperial forces "requestioned" them from the local civilians and retailers. As for the British - their troops were undertrained, their aircraft e.g. Brewster Buffaloes were obsolescent and some units lacked proper communications.
Not only that, some of the responses were poor. Singapore was hit by an air raid on 8 December and a blackout still wasn't ordered. Facilities were not properly destroyed when abandoned - the British failed to destroy the radio facility on Penang when they evacuated it on the 13th, so the Japanese got it intact four days later and used it to taunt the Singaporeans, asking "How do you like our bombing?". The troops then looted Penang and massacred the ethnically Chinese residents - a move that went too far even for the Japanese, who eventually executed three soldiers over it. General Wavell, in charge of the local area for the British, kept overruling tactical commander Lt. Gen Percival, but never removed him - a half-measure that just wrecked morale.
On 11 January 1942, the Japanese took Kuala Lumpur with limited difficulty and headed down the island towards Singapore despite the British blowing bridges as they went - that great British fortress was in sight. The British had suffered two divisions worth of casualties - the Japanese only five thousand men. The local civilians of course had it far worse - the precise numbers killed during the invasion and subsequent occupation is unknown due to the destruction of records, but was likely between twenty and fifty thousand.
Malaya was one massive defeat for the British - but worse was to follow.