Complex sentence: Within Burma, the Japanese disbanded the Burma Independence Army, which had grown rapidly during the Japanese invasion of Burma, but was only loosely organised and in some cases was opposed to Japanese control, replacing it with the Burma Defence Army, trained by Japanese officers.
Split into simpler sentences: Within Burma, the Japanese disbanded the Burma Independence Army, which had grown rapidly during the Japanese invasion of Burma, but was only loosely organised and in some cases was opposed to Japanese control. The Japanese replaced it with the Burma Defence Army, trained by Japanese officers.