 The Japanese army advanced swiftly southward through the Malay Peninsula , crushing or bypassing Allied resistance . 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 . 
