From Shanghai 1937: Stalingrad on the Yangtze, by Peter Harmsen (Casemate, 2015), Kindle Loc. 2002-2015:
At a deep cognitive level, there was a reason why Chiang Kai-shek and others around him wanted to believe that not just Soviet aid, but also direct Soviet participation in the hostilities was imminent. This was how they expected a war with Japan to pan out. The Chinese General Staff’s War Plan A, drafted in 1937, was based on the premise that a conflict with Japan would soon set off a larger conflict between Japan and either the Soviet Union or the United States. Therefore, the key aim for China was to hold out against the superior Japanese until it could be relieved by the arrival of a much more powerful ally, whether Russian or American. This plan was not as naive as it might seem, but was based on the calculation that neither Moscow nor Washington would want to see Japanese power grow too strong on the Asian mainland.
Some of Chiang’s commanders believed that it was partly in order to hasten outside intervention that the Chinese leader decided to make Shanghai a battlefield. It was true that Shanghai offered tactical advantages that the north Chinese plain did not, an argument that had been decisive in getting Chiang’s own generals to accept opening a new front there. However, these advantages would seem to be a small reward considering the risk involved in luring the enemy to occupy China’s most prosperous region. Much more crucially perhaps, Shanghai was an international city and a key asset for the world’s most powerful economies, who would not allow it to become Japanese territory, or so he believed. According to Li Zongren, one of China’s top generals, Chiang expanded the war to Shanghai because the importance of the city might lead to “mediation on the part of the European powers and the United States or even to their armed intervention.”