Plausible, but there really may not be a middle part longer than a few hours or days, depending on the mechanism (and I'm not even considering nuclear war here)

Plausible, but there really may not be a middle part longer than a few hours or days, depending on the mechanism (and I'm not even considering nuclear war here). I recall reading a first-hand account of surviving in Sarajevo during the war, and one of the things that most struck the author was how quickly -a mere couple of days- everything changed from going about the day as normal to apocalyptic survival-mode hellscape. And not because of specific battle events, but the breakdown of supply chains. We really have no way to predict it. It is even likely to be different in different locales.