"Trading space for time." After losing Shanghai and Nanjing, the Nationalist government retreated westward and made plans to wage a "protracted war" (chijiu zhan) across the vast expanse of central China. Attrition would wear down the Japanese in the long run. As Chiang Kai-shek himself explained in December 1937:

Appraising the outcome of hostilities, we are convinced that the present situation is favorable to China. The basis of China's future success in prolonged resistance is not found in Nanking, nor in the big cities, but in villages all over China and in the fixed determination of the people. The time must come when Japan's military strength will be completely exhausted, thus giving us ultimate victory.

Chiang had also hoped for direct foreign assistance in the early stages of the war, particularly from the Soviet Union and United States. Although both countries did provide material aid, this fell quite short of decisive military intervention that China needed. After Pearl Harbor, the United States agreed to help train and equip the Nationalist armies and then support thei efforts to retake occupied territories from the Japanese. However, the atomic bombings suddenly put an end to these initiatives.

This is a very simplified summary, but I think it addresses your question. I hope you find it helpful! :)