This is terrorism. This is what terrorists do.

It isn't a war — not black against blue, or us against them. When a gunman opened fire Thursday night on police officers, on our police officers, all of us were attacked.

Stunned witnesses described a peaceful march with about 800 protesters gathered in downtown Dallas to register sorrow and concern over police-involved shootings this week in Baton Rouge and suburban Minneapolis.

They were escorted by about 100 police officers, our officers, there to act as protectors.

"The cops were peaceful. They were taking pictures with us and everything," one marcher told KXAS-TV Channel 5.

"It was a lovely, peaceful march," said another.

The peace was shattered by terrorism, a well-planned and barbaric attack on that peace. In purposely murdering police officers, this gunmen did not further any cause or make any kind of statement that civilized people can understand. Fire was opened, literally, on human decency.

In the days ahead, some apologists — not many, but a few — will take the line that these murdered officers are casualties of war.

They'll say this act was the culmination of countless provocations and frustrations; they'll try to frame some kind of context around this chaos. They'll say, "It's terrible, but … "

No. It's just terrible. It's sickening, cowardly. It furthers no cause; it accomplishes nothing but misery and grief. It's violence for the deranged love of violence itself, disguised beneath a political veneer.

To employ a Texas colloquialism, it's chickens — with a gun.

City officials and police departments train and drill and try to anticipate disaster, but how can you anticipate determined inhumanity? You can't, not if you're human.

Mayor Mike Rawlings looked frazzled and stunned during a brief television appearance late Thursday.

"At 8:58, our worst nightmare happened," he said wearily. The cliché is justified because it's factual. This is a nightmare: The nightmare of a terrorist attack at a peaceful, familiar street intersection where we have all been a thousand times.

This is one of those moments that is going to test all of us. Dallas, our home, is about to become famous again for all the wrong reasons. It's going to be a shorthand for chaos.

We've been there before.

Today, we have a choice. We can let somebody else write the script, tell us that some of us are victims, and some are perpetrators; tell us which camps we're in and whose side we're supposed to be on.

Or we can unite in our shared shock and sorrow. We can all choose the side of decency and humanity. We can reject homegrown terrorism, mourn our fallen and support our wounded. We can wrap our arms around the community we share, and make our message clear:

Terrorism can hurt us. On Thursday night, it did.

But it doesn't win. This is our city, and we won't let it.