This answer from this slightly older thread should answer your question
Edit: I would recommend looking at the first answer for more information, but i'll just copypasta the more relevant part
Debated, sure, but it wasn't a serious, operational option at the time. The US tried its best to meditate from the start and even threatened to stop supporting KMT should Chiang not listen, and it was when Chiang was "winning" the war.
The story basically goes like this: at the beginning to mid phases of the Civil War, KMT, with support of US weapons and training, thrashed the CCP in the Northeast, thus forcing Mao to beg for a truce via Zhou Enlai's connection with the American. The American, with Marshall leading the mediation efforts, threatened to withdraw military and financial support and that made Chiang to stop his offensive against the wishes of many KMT generals. That truce lasted for a short time.
At the same time, Chiang began to alienate his supporters everywhere. The American delegation hated him, KMT cardes other than his own clique hated him, the general populace, university students hated him especially, because they wanted to form a coalition government with the KMT, CCP, and the Democratic League to stop the war. Chiang organized a National Assembly on New Constitution unilaterally without support from CCP and the Democratic League, and that divided the country even further.
At the same time, hyperinflation started to set in due to Chiang's single-minded pursuit and mismanagement of the war and basically botching everything. Corruption was deep within KMT ranks and Chiang was unable to do anything about it because those who were the most corrupted were from his clique. He also shelved the most effective general who was the main reason for their success in their earlier Northeastern offensives and replaced him with someone from his own clique, who would follow his orders to the tee and not command based on his own initiative.
The political situation was hopeless and when KMT started to lose, it was fast and big. Due to the rapid major defeats, Chiang was lobbying for more direct American involvement, and the US was worried too and there were talks that it was willing to give more direct aids under the condition that Chiang had to step down. It was said that they were actively looking for candidates for Chiang's replacement and settled for vice president Li Zongren, who could not accept that offer due to him being a former warlord and other KMT factions would not follow his order and Chiang's clique would impede his directions. It turned out to be a prophecy, as when Li finally took over later, Chiang's clique would just ignore his order and do what Chiang told them to do.
By the time Chiang had formally stepped down at the last stage of the war, everything was too late and KMT was too fragmented and hindered by factional politics that it could not even mount an effective defence along the Yangtze River. At that point, Chiang had already set course to flight to Taiwan, leaving Li Zongren and others to deal with the mess in the mainland.
Long story short, the US would not be able to intervene even if it wanted to. There was no reason for direct involvement because KMT was winning in the early stage, and it couldn't intervene in the later stage due to KMT losing too fast. Since KMT was losing popular and all kinds of support, the US would have to completely take over at least half of China and establish US military rule to handle the situation. It was politically and logistically impossible.
The American's quest to replace Chiang also didn't stop even after the KMT had fled to Taiwan. This time the candidate behind the scene was the aforementioned hero of the Northeast, Sun Liren. Chiang caught wind of it, acted proactively by arresting and accussing Sun of his role in a conspiracy to start a coup. Sun was under house arrest for life.