The fact that China despite being poor and weak was able to survive and resist Japan was literally the reason why Japan attacked Pearl Harbor.
Of all the Allies, China actually fought the longest, for fourteen long years. It had no industry to arm and supply its millions of soldiers, nor did they had the money to pay them. Japan ruled the skies, and their warships patrol China’s rivers from the Yangtze to the Pearl River.
And yet, China survived, its armies, greatly inferior they were compared to the enemy or any of its worldwide contemporaries, remained intact, victorious even.
While they have lost the battles at Shanghai, Nanjing and Wuhan, the Chinese then won battles at Taierzhuang, Wanjialing, Shanggao, and three times at Changsha. Mao’s Communists for their part undertook their own offensive involving hundred guerrilla regiments, destroying railroads and attacking behind enemy lines, further negatively impacting Japan’s invasion.
Their failing invasion of China, initially successful at first, has drained Japan’s resources for little to no gain. Worse, Western nations now refused to trade with them and provide them with the needed resources, such as scrap metal and oil. If Japan will not have these resources, not only their invasion would come to a stop, they might also be driven out of the country altogether. After all, the Chinese themselves have already did two successful all-out offensives entirely on their own: the Winter Offensive which made minor gains while also defeating Japanese forces in Henan and Suiyuan provinces, and the aforementioned Communist Hundred Regiments’ Offensive that degraded the ability of the Japanese to supply their forces in the field.
So Japan’s leaders were like “give us what we need and let us trade with you or we attack you” and when America refused, they then attacked Pearl Harbor. The Japanese thought that the Americans would simply give in to their demands, but no. They blundered themselves into fighting a multi-front war while still dealing with their bogged-down invasion of China.
During the closing months of the war, the Chinese are now actively engaged on a grand offensive aimed to wipe out the Japanese occupants and liberate the occupied territory. Two atom bombs dropped in the Japanese mainland that forced their emperor to announce surrender stopped them from continuing their offensive, as the invading Japanese forces surrendered en masse to the Chinese army anyway.
Chinese officers inspecting a surrendered Japanese unit before herding them to a prisoner of war camp, September 1945.
By the war’s end in 1945, China, having survived and remained in the field for 14 years, emerged as the strongest nation in Asia.