It is widely assumed that a conventional great power war is impossible in the nuclear era. The logic is clear: the stakes are simply too high for nuclear-armed states to make that kind of decision. Yet insecurity spirals have their own logic, and Washington should heed the lessons of history.

Although the Biden administration has been relatively careful and judicious in its approach to arming Ukraine, it may be rapidly approaching a more perilous period of this conflict. Ukrainian defenses have operated better than anticipated. Still, the odds are on Russia’s side, and Russian forces will likely seize more Ukrainian cities and inflict more harm on Ukrainian civilians, thus increasing moral outrage at Russia’s actions. Pressure is likely to mount for Western governments to offer additional assistance to Ukraine, particularly if an insurgency erupts in Russian-occupied areas.

The Biden administration should be extremely wary about responding to such pressure. In particular, arming and backing an insurgency would blur the line between being a supporting actor and being a co-combatant. The United States must also be prepared to restrain its allies. For example, it may be tempting for the countries closest to the conflict to consider unilateral steps such as supporting an insurgency or offering Ukrainian fighters safe haven on their territory. Nevertheless, it would not be out of line to make clear that the United States might interpret Article 5 commitments loosely in such instances, meaning that should Russia retaliate, the United States might not be obligated to respond with military force. Amid the present spiral, Biden and his team must determine where the United States’ own limits fall and focus on remaining well within them.