You wouldn't invade and occupy a city of over 10 million. An insurgency would threaten the city and the region and you would be invited in to help control it.

You wouldn't invade and occupy a city of over 10 million. An insurgency would threaten the city and the region and you would be invited in to help control it. Imagine 1% of that 10 million joined an insurgency. That is 100,000 people. I think this is a distinct enough possibility for a military to prepare for. In fact, the US already works on both ends. Supporting insurgencies in Libya and Syria while helping fight them in Iraq, Yemen, and Afghanistan. These aren't mega cities but the possibility still remains.