They forgave Japan because of CPC victory in 1949 and the Korean War, when Japan become a logistical supplier for UN troops in Korea. That’s when you have the San Francisco Treaty (in which both PRC and ROC refused to sign). The Truman Administration long hoped the ROC would America’s main bulwark in Asia against the Soviet Union (which already occupied northern Xinjiang and Manchuria in 1945). Call it history’s tragedy, but Chiang’s KMT was simply a failed state from 1944-1949. The 1944 Operation Ichi-Go drained Chiang of the last bits of tax revenues he received from agricultural tax in Hunan, Guangxi, and mother parts of China not occupied by Japan. Corruption at all levels then was the only way for KMT bureaucrats to survive. The rest, as we know, is history. But the result of the Chinese Civil War and Korean War is China was no longer a U.S. ally against the USSR, so postwar Japan replaced China. Things happened quite quickly, so there was no denazification in Japan. Washington needed a competent and internally stable ally capable of collaborating and coordinating with U.S. strategic goals on a broad range of issues. And Chiang’s China was too weak to play such a role. In short, neither the Cold War nor WWII (plus Chinese and Korean civil wars) ever ended in Asia. Chiang was a Shakespearean tragic figure who simply lacked the resource to fix a broken state. But Chiang did what he could. And because of this tragic post-war history, Japan was never punished for its crimes.