Many are asking: How can we stop Russian President Vladimir Putin from moving into Ukraine and seizing a large chunk of its territory in the east? The actions of forces that resemble the Russian special operations troops who created the conditions for annexation of Crimea suggest that other parts of Ukraine may also be in the Russian strongman’s sights.

The fact is, however, we cannot stop Putin. Or, to be more precise, we should not try to stop him physically. Doing so would require military threats or troop deployments to Ukraine. The stakes do not warrant such a step. It is not worth risking World War Three over this.

Ukraine is not a member of the North Atlantic Treaty Organization. It does not have a formal security treaty with the United States, and its strategic importance is not great enough to warrant such escalation. Though we can feel for Ukrainians — and reject what Putin is doing — this is a classic case of where the old axiom “We can’t be the world’s policeman” does apply.

Yet we cannot be indifferent to what happens in Ukraine. The stability of the international order has been compromised by Russia’s blatant aggression. The norm against interstate violence and forceful changing of borders has been violated.

In addition, we have a commitment, dating back to the 1994 Budapest declaration, to help Ukraine defend its national security. Under that agreement, Ukraine agreed to give up its nuclear weapons — which served the global non-proliferation agenda and U.S. interests. It also implied a moral commitment not to look away if Ukraine came under threat.