Competition for limited resources.
Water that doesn't glow in the dark, someone suggested toilet paper (which was hilarious considering how many rolls my wife picked up to tide us over during this whole COVID19 thing), food (that doesn't glow).
If the choices are:
- Let my kids die
- Kill some people and take their stuff
...a lot of "moral" and "reasonable" people will choose "kill and take". People get down right unreasonable when their childrens' lives are at stake. If COVID-19 were killing the youngest 2-4% instead of the oldest, peoples' reactions to it would be quite different I assure you. Mine certainly would be, I have two kids: 9 & 12.
They're side started it, and they need to pay for it, regardless of who "they" are, and any inconvenient facts that might get in the way of conquering the world.
Bat shit crazy
If your only supply of nonradioactive water has a little too much mercury in it, you end up with a society of Mad Hatters. One would have to call into question the ability of a city of lunatics with heavy metal poisoning to wage war, but you might be able to pull it off.
Or maybe the first generation or two had the metal poisoning problem, and the rest is just "the sins of the father" action on a society-wide scale. Sprinkle in some cannibalism during the worst of the madness times for spice, cook at 350F for 150 years, and serve!
Take the United States. England shipped all their religious whack-jobs off to start colonies. Then sprinkle on a hundred+ years of just about everyone with some bad luck and a willingness to take a leap of faith for a better life... tada! Modern USA. Still a non-trivial amount of religious whackos floating about, still down-right puritanical at times about sex & nudity.