This will require some mental gymnastics. But, then again, engaging in such eccentric exercises is what makes alternate history so much fun.
Let's make Henry Wallace the vice-presidential choice of the Democratic Party in 1944. Thus, when FDR goes on to his reward, Wallace becomes president.
Like Truman, Wallace views the atomic bomb as little more than a new kind of explosive. Thus, he authorizes its use against Hiroshima and Nagasaki. However, in the absence of a Soviet attack on Manchuria, Japanese military leaders in China denounce the surrender of their government and vow to continue the struggle.
Wallace refuses to deploy American forces against the Japanese hold-outs in China, but delivers copious amounts of aid to both the Nationalist and Communist forces. True to form, the Communists avoid all but the most perfunctory of operations against the Japanese, thereby accumulating large stocks of American arms and ammunition. When, in 1947 or so, the Nationalists finally defeat the Japanese hold-outs, they are exhausted.
Wallace calls for talks aimed at creating a coalition government. When the Nationalists refuse, Wallace cuts off aid.
The rest, as they say, is history.