Traditionally, China has always been invaded from the north, because that is where the most militarily defensible positions are. That is why the Great Wall was built there. If you read about the Battle of Shanghai, the Japanese Navy refused to take on Shanghai alone, and the Army had to get involved. That means if Chiang had met the IJA in the north, they would likely not have had enough manpower to take on Shanghai.
Again, I would refer you to the defense of the Great Wall. 300k NRA soldiers defended the Wall for months. Are you going to tell me that Chiang could get 300k men there, but couldn't send them enough bullets throughout months of battle? Forget tanks and howitzers, get those guys enough machine guns and mortars and they would've sent the mere 50k Japanese troops packing. Of course, that would've meant equipping troops that were not particularly loyal to Chiang (the reason they were sent north in the first place), and having the risk of Song Zheyuan going warlord.
You mentioned the Japanese having the advantage in the north because of their mobility and armor. Well yes, if by "north" you mean the Central Plains. That is why the Great Wall is crucial. The ROC was not the first Chinese "dynasty" to face an enemy with superior mobility. The Ming fought Mongols and Manchu, both of whom had a lot more cavalry than the Ming. There's a reason why, when Wu Sangui surrendered the Shanhai pass, the Qing were able to sweep south practically unopposed.
china was mainly invaded from the north because that's where the threat most often came from, be it the xiongnu, (han) the tujue, (tang) the khitans/mongols,(song) or the manchus; (ming) china's biggest threat always came from the nomadic horsemen of the northern steppes. (a dynamic shared with the romans) this is why the great wall was built where it is.
the manchus took a very long time to take all of china, but beijing is close to the great wall so it was quick to fall. whenever the nomadic horsemen are able to make it into china for long, it's always at a time when china is corrupt and weak due to mismanagement. the japanese invaded when they did for the same reason.
as a result of political insecurity and a fear of internal subversion, chiang made some of the same mistakes (not trusting capable generals, undersupplying distant border troops) that the song and ming did when they were weak and had to ceded territory. he was simply not a very charasmatic leader and did not have very loyal troops. compound that with the fact that 1930s china was politically fractured and that famine was sweeping the land, we can say that the kmt still managed to give the japanese one hell of a fight in alot of places.
i'd say the most defensible parts of china are the mountains of sichuan/hunan/guizhou. these were the treacherous and inhospitable backwaters that the imperial governments never quite had a good handle on and where bandits proliferated. mao was the only one able to eliminate the bandits of west hunan over the centuries and he only did it after the kmt retreated to taiwan.
if you read the romance of the three kingdoms, (三国演义) they talk about the mountain passes of sichuan as a land of ambushes.
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