Some blamed Chiang and the KMT for not doing enough to fight the invading Japanese forces during WW2 and as a result Japan was able to occupy huge territories in China causing misery to millions of Chinese people. At that time Chiang chose instead to preserve his armies for the fight against the Communist Red Army. He saw the CCP as a greater long term threat to his rule over China. Chiang even moved the capital to Chungking so as to escape bombing by Japanese air planes. Had Chiang enaged the Japanese directly with the help from the Americans history may not be the same ....
there was no way that China could outright defeat the Japanese forces which had vastly superior military equipments. Chiang's strategy of "trading space for time" and bogging the Japanese down in the vast terrains of China was the only realistic choice of fighting the war. the strategy was right, executed to near perfection and resulted in the eventual bitter victory. it was regrettable that large territories were conceded to the Japanese as a result of that and the people suffered badly.
Chiang did not really preserve his armies against the Chinese Communists then. for instance. the central government units of the 74th corps and Tan Enbo's troops fought across North to South to West of China during the 8 years. the CCP fabricated this wartime lie to slander the KMT then.
the US didn't help China much. the US gave little aid and assistance to China before or after Pearl Harbor. the Americans' policy was "Europe first. Asia later." the 50 billions lend-lease program to the allied nations from 1941-1945. Britain had 60%. Soviet Union had 20%. China had just 2% and 0.3% and 1% in 1943 and end of 1944 respectively. the US did not arm any Chinese troops in China till late 1944. China survived the 8 years long bitter war largely on her own with little insufficient aid from US and rest of allied nations.