With Russian provocations against NATO member states growing more frequent, the alliance urgently needs to develop a credible doctrine of punitive deterrence to guide its response to acts of aggression that fall below the threshold of war. Mere territorial defense is not enough.

VILNIUS—On September 23, just a week after NATO aircraft shot down a drone that had entered Lithuanian air space from Belarus, a Russian military helicopter flying out of Kaliningrad briefly breached Polish airspace. Neither intrusion was an isolated episode. A growing catalogue of security incidents shows that the Kremlin’s war of aggression against Ukraine is pressing ever more ominously on NATO’s borders.