People don't realize just how easy it is to put a city into siege... It's not like the medieval era where you need a standing army outside it.

No, all you need is to destroy some key overpasses/bridges along with key power substations and you've effectively crippled the entire city. Crash some simi-trucks on the few remaining major routes out and you've clogged any major supply/evac routes. A few snipers harassing anyone trying to repair the damage and you've got a perpetual siege that will strangle a city within a week.

Edit: Please note that I'm not advocating for anyone to do this but to get into people's heads that living in a big city is actually a trap if it came to open conflict.