Research for post-apocalypse

That's a bit too broad of a question. You might consider slicing it up into parts and asking about the ones you are stuck on.

When you say "incite humans into widespread violence", it sounds like they cause rioting and pandemonium, as opposed to, say, total nuclear devastation. If that's the case, it might be helpful to go take a look at a couple of things:

-Real or fictional examples of mob violence and hysteria
-Disaster relief efforts in real life- e.g. Katrina, the aftermath of the Iraq invasion, Fukushima (sp?) in Japan, the Flint crisis right now.
-Economic disasters, starting with the Great Depression

I would just start asking Google and Wikipedia for examples of all this stuff and read closely to see what types of coping behaviors and mechanisms get taken up when normal supply chains are disrupted (e.g. what's going on in Venezuela right now). There aren't a lot of examples of how society reacts to demonic sabotage in real life but if you take a few real-life examples of behaviors from other disasters and sprinkle them in, it will add that touch of realism you're looking for. E.g. one of the first things that almost always happens in real disasters is profiteering- people trying to make a quick buck by jacking up prices for water and food in the disaster zone.

Seconding everything Remora said. You might also want to research the 1992-1996 Bosnian War. Sarajevo went from a modern city that hosted the 1984 Winter Olympics to an active war zone that suffered the longest siege in the history of modern warfare. Its rebuilding since may also give you ideas.

Things would return to normal in a few months, except in places like Haiti, where things never were that good. Assuming that the infrastructure has been widely damaged and civil authorities have been hammered and the population has been cut by 90%, then it might take a year for things to be humming again. Humans are difficult to kill, and knowledge is widely dispersed, so things would just be put back in shape, and considering how necessary it would be it probably would be rebuilt with fewer hitches, than if it were being done now.

Third world countries often suffer the least from apocalyptic disasters. No technology to lose. Take a look at Vietnam post-US occupation. Took about a month for villagers to replant crops, rebuild housing and livestock pens and continue on with life as it always was. Even the issues with mines, unexploded munitions and damaged transportation lines had little effect. Central governments and societies that deal with them are far more affected by collapse than those whose lives were unaffected to begin with.

The Bhopal chemical plant explosion killed thousands. Locals buried the dead and went back to work. The 2004 earthquake and tsunami sent 100 foot waves over the province of Aceh in Sumatra, wiping out anything and anyone on the coast. A year later life was back to normal, other than some industries leaving the area. Take a look at genocides such as Pol Pot's regime in Cambodia and the East Pakistan/Bangladesh genocide that each killed hundreds of thousands to millions of people. The basic farmers and other subsistence occupations have changed little due to regime change.

Compare all this to Katrina, Germany after WWII, Japan after WWII, the Soviet States and so on and you'll see that local apocalypses (apocalypsi?) have far greater affect on more advanced countries. But then, since nobody in recorded history has faced a global plague of demonic origin and your technological time frame has yet to occur, you might have some latitude to fit your story into.

Jeff

I tell people interested in writing dystopian or post-apocalyptic fiction that they should check out and read Nothing to Envy. Not only do you get insight into life inside North Korea, but basically North Korea has been living the whole dystopian/post-apocalyptic thing. Though they preached self-reliance and how they don't need the help of other nations, the reality is that they were heavily dependent on the Soviet Union for handouts and once the USSR collapsed, the North Korean economy went into a freefall that it's never recovered from. It's a little better now, because China is giving them handouts, but when it comes to North Korea, their economy isn't in the toilet; it's in the ground beneath the toilet. That's how bad it is.

Aceh and the places affected by the 2004 earthquake didn't get back to 'normal' on their own. They received a great deal of assistance. And they certainly didn't do it in a year.

Ahhhh! Dystopian stories are my thing. I pondered your questions and have some ideas to toss back at you. First, even a pending war between angels and demons can't be kept secret from humans so you will have political leaders and countries taking sides. At this point, with allies established, it should degenerate into an actual war and not just a series of terrorist acts. Second, the countries that were spared the worst of the damage are most likely the countries that weren't the battlefields. Also, they might have had a better deal with their ally of either demon or angel. So, I would say your setting should look something like a world war with limited tactical nukes with most of the power grid and communications offline because of EMP weapons. Since the different sides are backed by supernatural entities, I would say that each side has at least one safe haven, a large city that has been protected by the supernatural powers of their ally.

I guess the critical question here is why are the demons and angels fighting and why did the demons make the first move? One would think there has been an uneasy truce between good and evil for thousands of years so what is the catalyst that's breaking the truce now? Forgive me, I'm not trying to rain on your idea, but you might want to check out one of the seasons of the TV series Supernatural. They did a whole theme on the truce being broken and demons and angels using Earth as their battleground. If you can take a plot idea that others have mined and put your own spin on it and make it your own then more power to you. Anyway, I hope my thoughts help. Good luck to your WIP.

'The world' is a very big place. I think you'd be better off focussing on your fictional 'major U.S. city', and only mention the rest of the world as fleetingly as possible.
I'm with Weaselfire in a general way: places without much modern technology will do much better than cities. People living 'off the grid', people living 'traditional' lives anywhere. People who never had much access to modern life.
A family in a village in rural India might not notice for a while that the electricity they only get for a couple of hours a night hasn't been on for a week.
The Amish farmer who only realises that there's a problem when he goes into town. (Or meets people fleeing the town.)
There are people herding sheep or plowing with water-buffalo who might not notice for even longer.

Depends on how the demons incite violence, as well. It's easy enough to start trouble in a big, crowded city. In a small town, or better yet, a village or family group, where people know each other, at least by sight, people are going to both cut each other a little more slack, and know the trouble-makers and sore spots.