Short answer: we did. The US government supplied a massive amount of armaments to the Nationalist government (which SecState Marshall pointed out that they promptly lost to the Communists.... on the order of 30-40 percent... his commentary was that if it rose to 50 percent, it would be a very bad idea to continue to supply them).

A good source to read on the topic would be David Halberstam's "The Coldest Winter: America and the Korean War" which has quite a bit of discussion about Sino-American policy in that time period.