According to whose rules?! The problem with trying to out-guess a nightmare is that the nightmare is undefined. You can't predict chaos.
For instance, in my state (Washington) Seattle is a lefty loony-bin literally surrounded on all sides by the right. In the event of a civil war smaller internecine warfare would develop between the two factions, drawn down ideological lines (as well as geographical ones).
Already surrounded, Seattle would be likely be quarantined. But that's all conjecture because we don't (can't) know how the dominoes will fall.
Blow the power grid, blow the water lines, blow any fuel supplies coming into the city. Blow rails, blow bridges.
Wait.
Modern people are not like people of 1860. The Cities of 1860 did not have so many serious vulnerabilities as do modern cities.