Vladimir Putin has surely changed Europe's security environment. Photographer: Mikhail Klimentyev/AFP/Getty Images
Vladimir Putin has surely changed Europe's security environment. Photographer: Mikhail Klimentyev/AFP/Getty Images

Say this for Russian President Vladimir Putin: He has ended NATO's decades-old existential crisis. When the North Atlantic Treaty Organization meets tomorrow in Wales, the abstract question of the alliance's purpose in a post-Soviet world won't be on the agenda. Instead its leaders must devise a plan to counter the very real threat from an irredentist Russia.

The only question they face is how far to go. The alliance has been divided over how sharply to respond to Russia's incursions into Ukraine, and the new NATO states that border Russia have begun to wonder whether the alliance will protect them. To prove their commitment to the alliance's collective security pledge, NATO leaders are expected to create a new rapid reaction force, rotating troops through Poland and the Baltic states.

This approach smartly puts a "tripwire" of alliance troops in the path of any Russian attack on these countries. At the same time, it avoids reversing the commitment NATO made to Russia in 1997 not to station permanent bases in central and eastern Europe. Rotation provides the option of leaving the bases equipped but empty, should Putin pull out of eastern Ukraine. That may seem a remote possibility now. But if the seven-point peace plan Putin proposed today proves to be a genuine effort to de-escalate the conflict, such flexibility may be useful.

NATO's leaders should show some resolve with their flexibility by going further and clearly stating that Russia has broken its side of the security agreements that were made at the end of the Cold War. These agreements include Russia's 1994 guarantee to Ukraine never to use force to threaten its territorial integrity, in exchange for Ukraine giving up its nuclear arsenal. The 1997 NATO-Russia Founding Act notably commits Russia to "refraining from the threat or use of force against each other as well as against any other state, its sovereignty, territorial integrity or political independence."

As for the act's pledge that NATO would not build permanent bases on the territory of its eastern members: The assurance was conditional on the continuation of "the current and foreseeable security environment." It almost goes without saying -- and the alliance should say so -- that Putin's annexation of Crimea and invasion of eastern Ukraine have profoundly changed the security environment that existed in 1997. Putin should be put on notice that unless he changes course, permanent bases will follow.

For their part, NATO's members have to spend more money; last year only four of the 28 members spent the 2 percent of their gross domestic product on defense they have agreed to spend. They also need to stop selling Putin the weapons and equipment that enable his aggression; France took an important step today when it finally agreed to suspend delivery of the first of two Mistral class helicopter carriers it had contracted to sell to Russia.

The Cold War that NATO was founded to fight ended a quarter-century ago. The new kind of war that has emerged since Putin began to challenge the borders around him is murkier and messier, lacking the harsh but clear lines of the Iron Curtain. In Tallinn today, U.S. President Barack Obama assured Estonians that NATO remains "the strongest military alliance the world has known." That claim will be tested in the months and years ahead.

To contact the senior editor responsible for Bloomberg View's editorials: David Shipley at davidshipley@bloomberg.net.