Although US intervention contributed greatly to China's eventual victory in the Sino-Japanese war, the situation in China and the nature of the conflict was not massively affected by the contribution, and in reality the intervention had little impact on the internal affairs and overall success of the second United Front. Both before and after US involvement tensions were never relieved between the CCP and the GMD and so Japan remained a superior fighting force. Peace between the CCP and the GMD was regularly encouraged by foreign powers and in China in order to fight the Japanese, but if events such as the Xian Incident had not happened, or foreign powers had backed one party to take control. The country may have been much more united and in a greater position to fight the Japanese, but as it stands China's contribution was not significant enough to account for Japan's defeat, the conflicts nature did not change, and without the forced surrender of Japan China was on the way to losing the war.
The early years of the Sino-Japanese war explain why the situation did not change after US intervention, and without these events the nature of the conflict for China could have been much more positive. An example of such an event is the Xian incident in 1936; Chiang Kai-Shek was kidnapped by Zhang Xueliang and put under house arrest, forcing to fight the Japanese instead of the CCP. The CCP before this point had just completed the long march. Due to this it is possible that the GMD would have defeated the CCP at this point. If this had happened, the military force in China would have been controlled by one constituency, which would have avoided internal conflict such as the Southern Anhui Incident in January 1941. Although desperate tactics such as the bursting of the dykes in the Yellow River did show weakness.