I was watching a show earlier that made me wonder: How was policing handled during times of war? I mean, I know that nowadays, the police force is largely unaffected by wartime, but back then, they pretty much sent all of their able men off to war. Unless I'm mistaken, women weren't generally trained to be police officers that early. So, how did it happen? Did police officers take turns policing and soldiering, did they draw names out of a hat, or were police officers just exempt from military service requirements?
You have organized crime, on one hand. I'm sure that not all of them honored their country's rules about sending able-bodied men to war. You'd have anarchy without an effective police force.
In WW1 Italy the Carabinieri acted both as Military Police/barrier troops, and a number of them was simply left behind in order to guarantee the rule of law. Both the Guardia di Finanza (which also dealt with border control) as well as the civilian Regia Guardia di PP. SS. (Royal Police) had no such luck and were sent en masse to the front.
Ernest Hemingway's A Farewell To Arms details the Carabinieri's role in the conflict fairly well.