PUT YOURSELF in the shoes of a police chief, and the impulse to allow a melee between rival ideological mobs in your city streets — when the alternative involves deploying officers between them, in harm’s way — becomes understandable. Understandable, but still not smart.

That was the decision taken last weekend by police in Portland, Ore., a beleaguered force that, by its own reckoning, has been the focus of what it called “violent actions” virtually every day since late May, when protests erupted following George Floyd’s death in police custody in Minneapolis.

Officers of the Portland Police Bureau stood by and watched as hundreds of far-right demonstrators, including members of Proud Boys, a violent white-nationalist group, squared off for more than two hours Saturday against left-wing and anti-racist protesters downtown. Officers did so as paintball rounds were fired, chemicals sprayed and rocks thrown while people swung at each other with fists, sticks and bats. In a statement, the police said they were badly outnumbered — just 30 officers were available for “crowd management,” against hundreds of activists — and they determined that intervening would not make things safer.

Besides, they suggested, in almost as many words, 80 straight days of hostility had sapped their will to interject themselves among “individuals . . . willingly engaging in physical confrontations for short durations.” Translation: If citizens want to transform a downtown street into a schoolyard brawl, be our guests.

Setting aside the buzz kill for Portland’s economic prospects, that’s not a wise move. Restraint is often the right stance for law enforcement, and yes, there are times when more assertive policing is fuel on the fire of unrest. The trouble with allowing public pandemonium is that it can invite more of the same, in one city after another, and function as an inducement to escalation. And the police in Portland have been allowing pandemonium for far too long.

We don’t pretend that the police in Portland have it easy, or that the judgment calls they are forced to make are clear-cut. No one can envy the ongoing chaos that has beset that city, in which President Trump, opportunistically, last month deployed badgeless federal forces to menace mainly peaceful protesters whom he smeared as anarchist mobs.

Still, law enforcement involves drawing lines to maintain order. Just as state and local police stood aside and failed to keep the peace three summers ago in Charlottesville, where a clash between mutually antagonistic groups ended in tragedy, the passivity of police in Portland was an abdication of responsibility. No one was killed or seriously hurt in Portland — this time. Next time might not go so well.

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