No Stupid Questions

Think of cops as the HR of the city/county/country. Wallmart HR can’t walk into Costco and apply their policies and reprimand someone working at Costco. In the same way, a NY cop can’t willy nilly walk in the streets of LA or Japan and arrest someone.

Usually those requests go through the state department and if the us has an extradition treaty, the extradition is handled by the country they’re residing in.

If there is none, it’s more murky and usually handled case by case. (Or just ignored.)

If we do the asshole thing and ignore all that, it’s going to be the military, not cops. (See Maduro’s “arrest”)

Generally, cops only have authority within specific jurisdictions, sheriff’s the counties they work in and cops the cities, though if there’s a justification, they can go outside that, but it’s kinda frowned on without that justification.

Cops travelling, like on vacation have no special powers. Their authority comes from the states they work in, not even the us federal government, and not from just being cops. Few countries would allow foreign cops to just operate like that. (Actually probably none.)

It’s for the best that American cops have no authority abroad. Imagine one of them choking to death a black man on the streets of London because the man was walking across the road

They're just tourists in other countries. Just like if a Japanese cop goes to visit the USA.

In the UK anybody can make a citizen's arrest if it isn't reasonably practicable for a constable to do so.

However it can only be for a crime that is indictable (can be tried at a crown court). So things like significant theft, burglary and assault.

A US cop has no additional rights to enforce the law than anyone else.

Cops aren't superhuman. They are people, just with extra rights given to them by some authority (a city, a district, a country). Their extra rights end where this authority ends.

Cops don't even have the ability to enforce laws of the United States outside their jurisdiction. That New York cop you mentioned couldn't arrest/detain/engage a criminal if they were on vacation in LA. Depending on the place some departments might allow it, but just as often they get annoyed.

There used to be a retired sheriff in my neighborhood growing up who maintained his peace officer license. He had lights and a siren on his personal vehicle and would pull people over and ticket them. The local court though would just dismiss everything he issued.

Abroad it would depend heavily on the location, but it seems unlikely many countries would tolerate a foreign law enforcement officer doing anything related to police work. There are all sorts of treaties and agreements about that sort of thing so likely a vacationing cop would create an international incident if they tried anything. I'm sure in some situations like murder or sexual assault they might get a pass, but it seems unlikely.

In Germany everybody, this includes foreigners, has the right to detain a criminal. But there are some things to consider. First you are endangering yourself by engaging with a criminal. Second if you are wrong and the person you are detaining hasn't committed a crime but, lets say, just a petty offence, you yourself can get in trouble for unlawful detainment and assault.

In Germany, anyone has the right to provisionally detain a perpetrator caught in flagranti until police arrives. This would also apply to the US sherrif on vacation.