I don’t have any general view of these situations, but in this instance it seems to me that Sergeant Crowley was justified in making the arrest. I don’t think it was simply to assert dominance (I’m not aware of conservatives arguing that — who are they?), but to bring about quiet and order. Gates was uncooperative and belligerent from the getgo, making racial accusations out of nothing, and stalling in providing proper identification. He gave the policeman a hard time, insulted the officer’s mother, proclaimed his own eminence and retaliatory powers, and so forth, all of which Crowley evidently endured. Then, when finally the policeman was satisfied and on his way out, Gates followed him to abuse him further and to continue to make a racial event out of it, alarming the onlookers gathered outside. Crowley did not want to leave the scene in such disorder, so he warned Gates twice and then finally arrested him.
Regarding whether officers have to accept abuse from citizens, there was an article in the Times saying that it varies — some say they take it, some don’t — and if a crowd gathers, that makes a difference. As one of the lawyers said at the Cambridge police press conference, there are different ways in which all things could be handled, but there was nothing illegitimate about what Crowley did. Yes, someone else might have done differently, but it is the policeman at the scene who must make instantaneous judgments. This was seconded by the black officer Leon Lashley, who was there and also saw Gates acting erratically, as did the Hispanic officer Figueroa.
What Gates faced is not comparable to what black men and black people in general face. Gates just lost his temper due to an inflated ego and perhaps fatigue, and then found he couldn’t bring himself to back down. It would be wrong to make this like the O. J. Simpson trial, in which many black people responded to the encouragment to see their own difficulties with the law as similar to Simpson’s, and got conned into supporting him despite clear evidence of his guilt, and then embarrassed themselves and the whole country by cheering wildly at the exoneration of a double murderer.