I doubt it's going to be a siege in the way we think or want. Most war today is fought in hearts and minds. If the left loses that, then the people still loyal will trap themselves in areas with people they agree with and kick out anyone they disagree with. DC is totally an example, and if they do not follow the will of the people or attempt to order around people who won't listen to them, then they will starve themselves. Trade will close down, and connections will falter. DC will go from Detroit 1969 to Detroit 1989 in 2 years.

The big battles will be the attempts to take over other neighborhoods. That will be small skirmishes at best and feel weird. Imagine constant attempts to create CHAS, and failing due to poverty and warring tribes inside. the best methods will be containment. When gas and food is a tenth of the price outside, there will be choices of what type of destruction to accept, and some may choose their own. The Japanese Last Man proclamation comes to mind. We then enter the streets and areas where there are vacuums