‘Duty to warn’ guided US advance warning of the Moscow attack. Adversaries don’t always listen

The U.S. warning to Russia couldn't have been plainer: Two weeks before the deadliest attack in Russia in years, Americans had publicly and privately advised President Vladimir Putin's government that “extremists” had “imminent plans” for just such slaughter.

The United States shared those advance intelligence indications under a tenet of the U.S. intelligence community called the “duty to warn," which obliges U.S. intelligence officials to lean toward sharing knowledge of a dire threat if conditions allow. That holds whether the targets are allies, adversaries or somewhere in between.

There's little sign Russia acted to try to head off Friday's attack at a concert hall on 's edge, which killed more than