I also thought that gang violence question was rhetorical. :P
Your prediction is "backed up" by disasters that happened in a time far too different from the modern day, and/or catastrophes that don't hold a candle to nuking the entire world. I would say the only one you could barely compare in terms of the scale would be the bubonic plague, but those have nothing in common other than a huge amount of deaths.
Oh, and the Holocaust too, but it's pretty hard to compare massive genocide in pseudo-secrecy to blatant nuclear world's end in anything else but, again, death count.
Clap on the back for writing this much
Brawl 2192 8924 8771
In this retrospect, since the top dogs of all countries are doing nothing but bunking down and protecting themselves, gangs are free to do what they want (except in Texas), if those countries have gangs.
I'd certainly say this is the weakest part of my theory on modern civilization after a nuclear apocalypse, honestly... If there's anyone with firearms as a part of their life that wouldn't stand for such selfish abandon, it'd probably be idealistic army men who sure as shit don't fight like militia dogs for their employers whom leash them in the first place. At the same time, you need to wonder if there'd be anything they could do to rebel since transportation is going to be limited to whatever isn't fried by EMPs.