(REUTERS/Jim Young)

Guns change the equation in so many ways. They make it harder for police to retreat, and more likely that a stand-off that might have been resolved peacefully will escalate. They make it harder for police to give suspects the benefit of the doubt, and more likely that a suspected criminal may not deserve it.

They make it easier for a mentally ill man to forever alter two families' lives in the name of "revenge."

After the killing of New York police officers Wenjian Liu and Rafael Ramos over the weekend, it feels perhaps more satisfying to place blame elsewhere: on protesterswho've cried for better policing, on public officials who acknowledge that the protesters' grievances are valid. But both claims deflect attention toward a vague culprit — "anti-police rhetoric" — and away from a more concrete and systemic one: the ever-presence and easy availability of guns.

Ismaaiyl Brinsley killed Liu and Ramos with a semi-automatic pistol. NYPD Officials couldn't immediately determine how Brinsley had obtained the gun — only that it was purchased at a Georgia pawn shop by another man more than 15 years ago. We also know that Brinsley was previously convicted in Georgia and sentenced to two years in prison for illegal gun possession.

Today we should be talking about the guns not simply because one was used Saturday in the shooting deaths of two police officers, but because guns underlie the very tension between police and communities in America that voices saner than Brinsley have been trying to resolve.

In other countries, homicides, police shootings, shootings by police, and gun violence are much more rare. It's more rare that patrolmen even carry guns. It's more rare that the civilians they encounter will be carrying one, too. In this country, by contrast, the ubiquity of firearms — the possibility of a gun, legal or illegal, in any coat pocket or waist band — injects a level of tension into police encounters that may be hard to entirely disarm even with the most thoughtful community policing reforms.

In the United States, the ever-presence of guns makes it seem plausible that a 12-year-old boy handling a toy might actually possess one. And it makes it more likely that an officer responding to him would pull his own trigger. The ever-presence of guns also makes it plausible that an officer interacting with a teenager might fear for his life — and act in that fear. And it makes it plausible — even responsible — that communities who often encounter law enforcement feel they must teach their sons how to respond to policemen capable of killing them.

In comparing American police tactics and relations to other countries, it's hard to separate the role of guns here from all of the mistrust, defensiveness and aggression that arise around them.

"There’s not a big gun culture in Australia," Geoffrey Alpert, a professor at the University of South Carolina who has studied police use of force there, recently told me. "So the cops don’t have to worry the way our cops do. There’s not always a gun in every encounter. They don’t have to think about that."

They're freer to retreat, to reassess, to leave their own weapons holstered.

This doesn't mean that we can't ever improve police tactics in a county where guns are commonplace. Alpert believes policing reforms are possible and worth pursuing. But this does mean that we can't really address police-community relations without talking about the fear of guns tugging at both sides — andhow guns make the job of policing that much harder, how guns fatally narrow the margin of error for poor policework, how guns turn misunderstandings, mental illness and suspicion into something terribly deadly.