Why have the Japanese defeated us ?
The Japanese had beaten us in Malaya, because they were better in all. At first they were good prepared on the warfare under those climatic circumstances as we found on the Malayan peninsula.
They had lighter uniforms with shorts and boots with rubber soles. In colour and cut those uniforms sometimes differed, but all were in a good camouflage style crediting to the jungle green. They wear the most differing types of headgear, cotton caps, steelhelmets or hats which they collected from the captured and dead. They even wore Malayan sarongs, two prisoners which we captured at Batu Pahat were fully dressed as Chinese kulis. This all has made irritations amongst our soldiers, especially because this was a land in which they simply couldn't distingish between enemies and allies. Those who observed the battlefields the first time listened to rumours about "fifth columns". So more and more British were convinced that the whole domestic population was fighting against them, because they couldn't distingish Japanese from Chinese and Malayians.
In terms of food the Japanese were better too. Because their main food was rice, they easily lived from that, which was enough to them. The British units had to transport their various goods on long and winding paths. The Japanese soldier had with him a can of water, cooked rice and some salted dry fish, which lasts for three or four days.
Almost all Japanese were armed with machine pistols or other automatic weapons, which were very suitable for hand-to-hand fighting in the jungle. Ours were usually armed with too heavy rifles and only few of them had automatic weapons. Most of our officers were convinced until the end of the fighting, that the exact firing rifle is superior to the inaccurate machine pistols, but to me this is not valid in jungle fighting, because shooting at long distances is not possible here. The Japanese had with them 6-8 ammunition magazines for every automatic weapon, all other ammunition was carried behind them by ordinary bicycles through the narrow jungle paths.
Beside their machine-guns their favourite weapon was the 50 mm mortar, which could be carried easily by two men. Artillery almost wasn't used until the siege of Singapore. For fighting in close terrain, hand grenades were also used in great numbers. Sometimes they climbed on trees and threw the grenades down on our vehicles. Very dangerous too were their sharpshooters, well hidden in the woods.
Perhaps the greatest surprise was the intense use of bicycles which they brought with them or had confiscated or bought from the domestic inhabitants, which were paid occasionally with banknotes, which were specially issued for the Malayan campaign. In colour and size they were similar to the pound notes of that time which certainly secured a lot of irritations among the population. Also this indicates how well the Japanese were prepared for this war. With bicycles they raised their speed and agility in a way which enabled their troops to advance much faster.
The British were far from being so mobile. This is commented by a look at our soldiera on the way to the battlefield, who had to carry his knapsack, steel helmet, gasmask, large boots, tent, rifle, bayonet, not to speak of ammunition. Food and other stuff were transported by vehicles, but what that means under jungle conditions you can easily imagine. Even credited to the fact that they were good motorized, our units were condemned to stand still.
Also the Japanese tactics were well considered. Small groups of men and
even to every man, were shown their targets kilometres away, to which
they have to fight their way through. These small groups were marching
through the jungle to clear up enemy positions, following enemies'
tracks, and finally to unify themselves with greater units. To stay
in contact with the supply units in their rear they established
wireless stations. When they met a far stronger enemy they avoid an
encounter, left him in their rear, and marched onwards. This tactic was
not only ordered for small platoons, but even for brigades and
divisions. It was successful mainly to the fact that the Japanese were
thoroughly trained in jungle warfare and instructed in measures and
structure of the terrain on their way to their targets.
(Gordon Lee, Battle for Malaya)