The United States actually agreed to terms other than unconditional surrender with Japan. Immediately after we dropped the first nuclear bomb we were negotiating with Japan but they refused to surrender unless the sanctity of the emperor was recognized. There's quite a lot of confusion about exactly how high that issue was kicked up the US chain. After we dropped the second nuclear bomb Truman was told that Japan would be willing to surrender unconditionally, but only with the secret condition that the emperor be allowed to maintain presence. Truman agreed quickly and the rest = history.