You forgot that the Soviets supported the KMT over the CPC until much later on.
The U.S. supported the Japanese invasion of China until the Japanese refused to support the Germans in Operation Barbarossa.
That was when the U.S. decided that the Japanese were no longer useful, so they cut off oil and scrap exports to Japan.
The U.S. then backstabbed their Japanese allies and switched sides to back the KMT for future use against the Soviets.
Teddy Roosevelt explicitly negotiated the end of the Russo-Japanese war in a way to give Japan a beachhead to invade China.
Teddy Roosevelt's plan was to have the Japanese Genocide all the Chinese and become the U.S.'s premier puppet in Asia.
Teddy Roosevelt's plan got superseded when the Japanese proved unable and unwilling to support the U.S. ally Germany in Operation Barbarossa.
The U.S.'s goal in WWII was to steal the European's Colonies (Except France's) through the use of their ideological brethren, the Nazi Germans and the Imperial Japanese.
The U.S. backstabbed Japan before backstabbing Germany once the Germans proved unable to complete their invasion of the Soviet Union.
At that point, the U.S. and the British had to save as much Nazi Lands as possible from Soviet Occupation, which was the reason for Operation Overlord.
The KMT was always weak and decadent, which means the U.S. would likely have abandoned them once they forced the KMT to invade the Soviet Union after WWII and failed.