The Chinese initiated the second Sino-Japanese war before they were ready.
The Nationalists were modernizing their forces with German assistance while at the same time hunting down the communists. The communists got a lucky break with the kidnapping of Chiang Kai-shek at the Xi'an incident in which he was pressured into stopping his hunt on the communists and to form a 2nd united front. This was not to just fight against Japanese. It was also to save the communists. So the Chinese get trigger jumpy and shoot first at the Marco Polo bridge. Then a few weeks later, they move into Shanghai and attack the Japanese settlement with aim of overrunning the Japanese garraison within a week. Talks to calm things down fail. However as stated, the Chinese were not ready for the large scale war that they got themselves in. Japanese make a counter attack, achieve victory in all of Shanghai, and make a fast press towards Nanking with the aim of capturing the then capital of the Nationalists would convince them to accept terms to end the fighting. Very lenient terms were issued to the Nationalists to surrender before the assualt on Nanking. The Chinese refuse. Chiang orders the Chinese defending Nanking to fight to the bitter end dispite the bad strategic geographical situation Nanking was in for defending. The assualt begins, defenses crumble, Chinese commander and a few others make an escape at the very end. Other remaining Chinese soldiers don't surrender but scattered into the population. Japanese put Nanking in 6 week lockdown and massacre fighting age young men. Rape and pillaging also occur. Japanese renew order to its troops to not do that to get local Chinese cooperation. Wang Jingwei joins the Japanese as leader of new Chinese government in formation, coming into full official existance by 1940.
The Japanese "aggressive invasion" is labeled as such because the Chinese were not ready for a total war but they initiated it anyway because the Chinese communists existence needed that change in orientation by the Nationalists for its own survival. Had the Chinese been ready, then there wouldn't have been a Japanese deep penetration and occupation of much of the central China mainland area. That invasion was not initiated by the Japanese. The idea that the Japanese just blitz China like Germany did to Poland is incorrect. To continue the story.. a few years later but in the similar dynamic, the Nationalists needed US involvement to be saved and the US decided to put the survival of the Nationalists central to its Asia policy by March 1941. The Nationalists were tempoarily saved at the expense of granting the US much greater position of influence in the Asia region. But to the US bewelderment, the Nationalists still lost to the Chinese communists as the communists got much needed time for replensihment and big gains as the Soviet Union gave Manchuria to the communists.
