[narag]

The military may have laws/rules to report misconduct, but structurally, units are incentivized to hide misconduct

The key word seems to be structurally. The US have this much copied feature of decentralization. Every town elects his sheriff, judges, a lot of the administration is local, then statal. Federal bodies are a far away, mistruted entities. Police is local.

It made sense for a huge country developed at the rhythm of railroad. Law and order must exist near the place where the crime is. I'm not so sure that it's still the case with current connected world. Maybe the US still needs a level more decentralization than most other countries. But if you look at other countries where police is less corrupt and citizens trust them, it's usually an entity dependent on central government, not the city council.

Making the investigative entity as far as possible from the investigated is much better for imparciality. Also involving judges, not in the same branch. Local police here has limited competences. Any serious crime goes to national corps. Police needs to go to national academy and get certified for the whole country.

In my country I would trust police much more than the military.

[rclayton]

That’s a fascinating idea, but in practice I think we Americans have completely messed it up. Some U.S. states like Georgia centralize investigative responsibilities (often when municipalities are too small to budget these services). This strategy hasn’t lead to great results. The Georgia Bureau of Investigation is notorious for falsely indicting and imprisoning minorities. I suspect the same is probably true in other states.

We do still have the FBI and federal Dept of Justice that is supposed to provide some semblance of oversight. However, as we have seen over the last 4 years, they are equally susceptible to political influence as local law enforcement agencies.

[narag]

There is an interesting fact that's seldom overlooked over here. We have comunidades autónomas, that might be similar to states, only smaller. They're relatively new, less than 50 y.o. and accumulates a disproportionate number of corruption cases. Why is that?

Central government was developed around the statute of the public officer. In the 1800s there was an unending flow of public officers in and out (the "cesantías") caused by political parties putting their people in public jobs and firing the others' people when every elections turnover. At a certain moment a system of merit access was imposed to provide stability. If you were under the line that divides technical from political, you're safe from firing. Still it's possible that you want to cooperate for promotions but, at a certain level, you just don't care.

Comunidades were developed from scratch, with a hight proportion of "external" workers (that don't have the protections of the public officer statute), with "merit points" distorting the exams (being the merit having been working for years without exams because you're friends with someone) and with very little judicial oversight.

TL;DR: to avoid local corruption, make the police come from as far above as possible. To avoid central corruption, make technical lead not political and involve judges not elected by politicians. Everything elected by politicians gets corrupted.