LOL
Yep - like collim1 said, we are not equipped or trained for that kind of thing, contrary to what the SJWs think. We mostly wear easy to spot uniforms, drive easy to spot cars, and carry light weapons. We would be no better off than anyone else armed with an AR15 and their own personally owned tactical equipment. There would be no “holding off” any determined modern mechanized force. You could take the largest police departments in the country, put them all out and they would get rolled over by a tank company or mechanized infantry company. Pistols, light rifles and some sniper rifles (even the .50BMG ones) are not gonna do jack to AFVs, artillery, and air.
Realistically, I think our primary role at first would be to facilitate the evacuation of populated areas near the combat zone, and traffic control. Maybe the protection of Red Cross aid, etc. If Russians or North Koreans invaded, you sure as hell would not want to be captured as a police officer / government official. Beyond that, if you were the Red Dawn type, you could do Red Dawn stuff with people of a like mind. I guess honestly, the most useful thing a cop could do would serve as a local guide and information resource for the military guys coming back to liberate it. That of course is for the guys who didn’t just skedaddle out and leave with their families, etc.
Be a fun book to write/read!
ETA - that’s all assuming that N. Korea or Russia could ever get the resources together to do something of that scale, which I highly doubt. Even if they did, this place is way too big to completely occupy.