Attacking Singapore first would not automatically solve Japan's wider strategic problems. In a 1939 war without the European emergency, Britain could send more of its best available forces east, including the British Expeditionary Force and stronger formations from the Indian Army. Those troops would not yet be committed to France, North Africa, Iran, or other wartime theaters. Defenders also need fewer troops and less preparation than an amphibious attacker assembling over long distances. The timing changes Singapore from an isolated target into a base likely to be reinforced before Japan can mount a major assault.
Notable Point · 34:01-39:56
Why Singapore is not Japan's easy opening target
An immediate attack on Singapore would face stronger 1939 British and Indian reinforcements than Japan encountered in the later war.