Complex sentence: The skill with which Japanese commanders maneuvered their mechanized infantry divisions was then best seen in Malaya, where the lighter weight of Japanese medium tanks allowed for a rapid ground advance so heavily supported by armor that British defenders never had a chance to establish effective defense lines.
Split into simpler sentences: The skill with which Japanese commanders maneuvered their mechanized infantry divisions was best seen in Malaya. Japanese units moved so fast, and were so heavily supported by armor, that British defenders never had a chance to establish effective defense lines.