You're right from a strictly tactical point of view. However, the generals defending the norther borders were all loyal to the Ming Emperor. How each of them would have reacted to Li overthrowing Ming is difficult to say.
I'll have to disagree with that. If you look at the map and see where Manchuria is with respect to Beijing, you'll know that ShanHai Pass is the shortest route to Beijing. Like I said earlier, going through others places may take longer. So this is a classic example of space vs. time. Wu and Manchurians didn't give Li any time to respond when they went through Shanhai Pass and Li was caught completely off-guard. If they used another Pass and even if the generals guarding those passes defected, Li would have enough time to shore up the defense since it would take more time for them to march on Beijing even if unimpeded. A good example would be what Jiang Jieshi did in WWII. If Japanese directly attacked Nanjing without going through the entire northern China, Jiang wouldn't have time to gather forces and Chinese govn't would be destroyed completely. In the end, the Japanese had to spend 2 years fighting through Northern China and this gave Jiang enough time to gather forces in Wuhan and Changsha. And Japanese was held in those places and fought to a stalemate. That was pretty much how China survived WWII. If Manchurians went through other places, the same t hing might have happened.
At the very least, Wu Sangui defected to the Qing for very good reasons. Li stole his favorite concubine, which is a clear indication that Li did not regard Wu very highly.
Chen Yuanyuan was not Wu's concubine. She was a high-class prostitute whom Wu visited frequently when he was in Beijing. I think it is simply a myth that Wu defected because of a prostitute. That is a standard excuse in Chinese history to explain why some man become evil. Oh, he was seduced by a girl... A man with Wu's stature could get any woman he wanted. He would not care about a girl no matter how beautiful she might be if she interferes with his political ambition, especially if that girl was a prostitute. We now think of a man-woman relationship in ancient times using our modern values. However, people in ancient times had a different idea on these things. It is common to hear that a man gives his favorite concubine to another man as a gift. The mother of the first emperor of China (Shi Huangdi) used to be a concubine of another man and was given as a gift to Shi Huangdi's father.
So it would not be hard to imagine that Wu would've simply give Chen away if it suited his plans. That's IF Chen was his to give away. The fact is Chen was a prostitute and was never a private property of any single man. Since she was an extremely famous high class prostitute, it was mostly likely that many powerful men had gotten her service. I am sure Wu was perfectly aware of that. If he was not upset about her seeing those other men, why would he be upset about one of Li's generals stealing her?
It is difficult to say that had Wu not betrayed his post, other commanders would not have done so either. In fact, that's probably what Wu was thinking: if he didn't accept the Qing's offer, some other general might accept it, and he would become useless.
Yes, I agree. Other generals might defect as well. However like I said earlier, if Manchurians went through other places, it would take much longer to get to Beijing. This would give Li enough time to gather his forces and fight. Space vs. time...
Further, there really is no comparison between Ming border troops, and those in the interior. By all account, interior troops were utterly corrupt and led by mostly incompetent commanders. That's part of why Li was so successful in his campaigns.
I think it is most likely that the border troops were much more capable than interior ones. Why? Interior troops were not exposed to serious battles. The most they've seen was some local bandits. Their fighting ability was limited, to say the least. Border units, however, had been fighting many strong and capable enemies for a long time, like the Mongolians, Manchurians and even the Russians. In fact, historically throughout the history of China, the units stationed along the Northern border had always been the most capable units since they had to fight very strong enemies, Huns, West Xia, Mongolians, Liao, Jin, Manchurians, etc. Since the fighting had been almost non-stop, these troops were all battle-hardened and extremely capable. If you notice, while many wars had been fought throughout China, most of the famous generals in Chinese history are those who were stationed on the northern border. This simply cannot be said of the interior troops who were most likely spending their time in the cities bullying harmless civilians. If you look at Chinese history, most of the peasant rebellions started in the interior and in most cases, border units were not in the fight. And in most cases, the leaders of the rebellion would court with the generals on the border to make sure they don't fight for the old govn't. And in most cases, these generals were granted lordship and autonomy in the new govn't in exchange for their promise not to fight for the old govn't. So you can see that these troops were feared by even the lawless rebels.
In addition, there is every indication that Li thought his work was done once Beijing was taken. He even proceeded to order the death of his best advisor, Li Yan. THAT is not the sign of a competent general who was ready to take on foreign invaders!
This has been what most of the new emperors did when they won the war. Song's Zhao Kuanyin did it in the most civil manner by asking every single of his generals giving up their positions and troops in exchange for monetary wealth. Of course, the hidden message was "if you don't, I'll kill you." Ming's Zhu Yuanzhang did it in the most brutal fashion by inviting all his generals to a banquet and blowing up the whole palace and killing every one of them. Yet, most of these emperors were seen as highly capable.
Ehhh... Keep in mind that Qing destroyed the cream of the crop of Ming armies some decades back, and the Ming were reduced to only being able to defend.
Yes, I agree with you . However, Qing also suffered heavily in the hands of Ming troops. Their first emperor was killed in combat.
Additionally, it has been a strategy used by China ever since the beginning of China that Chinese troops on the northern border mainly defend, not attacking. This is because most of the enemies China had on its northern border were nomads who can disappear into the desert/plain in no time and were nowhere to be found. IT became very costly to send expeditions to clean them out. So ever since Zhou dynasty, Chinese troops on the northern border had been mainly defending their positions, instead of attacking. This was also why the great wall was built (to defend, not attack).