Once the cycle of escalation starts, it’s hard to stop.

So it’s both alarming and unsurprising that Russian leader Vladimir Putin on Sunday put his country’s nuclear forces on alert, pushing back against the flood of sanctions and angry rhetoric from Western leaders that has followed his decision to invade Ukraine. “Top officials in leading NATO countries have allowed themselves to make aggressive comments about our country,” Putin said. He might have been referring to last week’s comment from the French foreign minister that “the Atlantic alliance is a nuclear alliance,” which itself was a response to Putin’s own nuclear-tinged warning against outside countries interfering in his war. Every tit-for-tat heating up of rhetoric just ratchets the tensions a little bit higher.

Escalating is easy. Prudence is difficult. But prudence is exactly what is needed from U.S. and European leaders in the days and weeks ahead.

What does that mean in this case? It doesn’t mean surrendering to Putin’s aggression by giving up sanctions against Russia or the (so-far) limited efforts to aid Ukraine in its defense. But it does mean remembering — as if he’d let us forget — that Putin has command of a large arsenal of nuclear weapons, and that any direct confrontation between NATO and Russian forces might turn a regional calamity into a worldwide disaster. It means (as my colleague Damon Linker put it) being very careful that the “urge to do something” doesn’t make a bad situation much worse.

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