Truman gets credit for the China monster. The Chinese had spies in his administration , so they knew in advance nuclear weapons would not be used. If that had remained uncertain Korea would be United and free.
Actually, not even Truman was responsible for China going Communist (let alone Nixon), though he certainly didn’t help with the overall situation. That blame can be placed squarely on FDR’s shoulders via the Yalta Agreement (not to mention Alger Hiss and his VP at the time, the communist one that came this close to getting the Presidency), and far more directly on two people: The first being Joseph Stilwell, a man who deliberately and knowingly handed the Communists China during World War II and sabotaged any chance of Chiang Kai-Shek keeping China firmly anti-Communist, basically being the John Kerry before Kerry was even born, and the second, surprisingly enough, was George Marshall (yes, the same guy who made the Marshall Plan), who due to naivety and partly because the Yalta Agreement basically tied his hands behind his back, let China go Communist. Had it not been for Stilwell’s siding with Mao Zedong out of explicit love for Communists, at the very least, China would be a solid ally against Soviet Russia and more importantly not Communist, not to mention most of Asia wouldn’t be communist either. Heck, we most certainly wouldn’t have had a Cold War to fight AGAINST Communism. In fact, Marshall and especially Stilwell’s screw-up was so massive that not only did that result in Joseph McCarthy calling them out on it, but even John F. Kennedy calling them out on it as well (and bear in mind, JFK is a Democrat, a fairly liberal one at that, so when even JFK realized just how bad that was, you KNOW it’s very bad.).