Right at the moment when Chiang Kai-Shek's Nationalists were about to win the Chinese Civil War (they had captured the Communist capital of Yan'an), the Truman administration demanded that they form a "power-sharing" government with Mao Zedong and the Communists. After Chiang quite rightly refused, the Truman administration imposed an arms and ammunition embargo on the Nationalists.
The Truman administration is directly responsible for the deaths of a good 60 million people or so. And of course condemning most Chinese (and by extension North Koreans, and Vietnamese, Cambodians, and Laotians) to live in brutal, abject poverty to this day when they could've been a first world country like Taiwan, South Korea or Japan.
Somewhat less clear-cut, but he also allowed Stalin to enslave Eastern Europe while we had the nuclear bomb and he didn't.
