He was the leader of an unruly coalition of "Warlords", some little more than bandits, who had misruled China since 1911. His control over the warlords was unsteady, his armies were usually badly equipped, badly trained and badly led and his military skill was dubious. In addition, he felt that the Communists, who had been his principal enemies before the war, would attack him again, as soon as the Japanese were defeated. Chiang's skill lay in the political arena, where he not only managed to stay on top of a cut-throat coalition of rival warlords, but also succeeded in selling himself as a pillar of democracy to the American public.
Massive U.S. aid, including the American Volunteer Group (or "Flying Tigers") led by Claire Chennault, was sent to Chiang, along with the highly talented (and highly abrasive) General Joseph "Vinegar Joe" Stillwell to help him push the Japanese out of China. Chiang, however, used most of this to prepare for war with the Communists, and only lost more ground to the Japanese. (Of course, the Communists DID attack him, and win the 1945-49 Chinese Civil War, so they probably WERE the greater threat to his forces).