Across Europe, Russia has been waging a quiet war — not with tanks and missiles, but with sabotage, cyberattacks, disinformation, and the slow erosion of institutional trust. NATO, built for a different kind of confrontation, finds its deterrence architecture ill-suited to threats that are real in consequence but ambiguous in form. The alliance now grapples with one of the oldest dilemmas in statecraft: how to respond firmly enough to restore credibility without provoking the very escalation it seeks to prevent.