doofus47 raises a valid point.
I am as pro-police as anybody I know. A lot of my friends are cops (or former FBI or former Customs). I have a lot of respect for good cops.
But while doofus47 is invoking officers or departments who are now infamous for abuses against minorities, there have been other institutional flaws at times, too.
An example that leaps immediately to mind is the post-WWI period, when the US government and big business were scared, in virtually equal measure, of the communist and anarchist movements.
In the Boston area, multiple police departments were co-opted into effective goon squads, to literally break heads at picket lines.
There weren't that many black laborers in the area at that time, but BPD and other departments' officers severely injured a lot of Italian, Irish, and Polish workers, under color of authority.
Every once in a while, we do in fact have tyrannical episodes in our own country.
I'm pretty sure the Wounded Knee, and other incidents, might have been viewed as tyrannical too, at least by some segments of the population. (The Army, in that case, not a police force.)
Go back further, and we have the Trail of Tears. In the events that ultimately led to that move (under Van Buren), the previous Executive (Jackson) ignored the Judicial ("John Marshall has made his decision; now let him enforce it!").
We seem to have come a long way since then... and even since the days of McCarthyism. But we may not be as far removed from potential tyranny as we would like to believe.
Regards,
M