All out shooting wars are one thing, being an occupying force is another experience altogether. It's one thing to go out on patrol and expect to see a uniformed enemy and have a shooting war - you're expecting to meet an enemy and have a proper battle. It's a whole other experience to run checkpoints in an occupied country where any car coming up to you could be loaded with explosives. It's this uncertainty that leads to frayed nerves, and sometimes really bad shit happens when you put nervous people with guns in a country with an insurgency.
I'm sure some inner city cops have seen more urban warfare than some returning war veterans.
I think a big issue is that being an antisocial really mean probably raging alcoholic shithead who doesn't do your job right and inappropriately escalates situations kind of guy will get you fired from just about any job, especially if you are interacting with lots of people in stressful situations daily.
But cops who fuck up and do their jobs wrong end up being "internally investigated" instead of simply fired and if laws are broken, immediately charged with crimes. Fuck you bet some shit company like Wal Mart will file charges if some employee steals or physically assaults troublesome customers.
American police forces are run like a network of decentralized paramilitaries that are accountable to no one and effectively operate outside the law, believing they simply are instruments of the law instead of citizens who should be real scared of breaking them.
Civilian led federally funded oversight committees with the power to order anyone including the chief of police to resign and face charges in court with a and a mass-culling of shitheads to nuke out destructive "cop cultures" are kinds of reform I'd like to see, but lol at that..