“We agreed today that [Ukraine and Georgia] will become members of NATO.” Bucharest Summit Declaration, April 2008

Nobody knows how the current phase of Russia’s war in Ukraine will end, whether with more, most, or even all of Ukraine’s territory liberated, or in an ugly ceasefire with Russia occupying significant parts of Ukrainian territory. The former seems more likely. In any case, the United States and Europe need to face a hard truth: when this phase of the war ends and Ukraine is left in a grey zone of ambiguity and insecurity, Russian President Vladimir Putin ultimately could resume his war of imperial conquest and national extermination at will.

As NATO prepares for its summit this year in Vilnius, Lithuania, in July, the alliance needs to look again at membership for Ukraine as the best way to lock in and enforce peace in Europe, a benefit that would far outweigh the costs. Contrary to much either-or handwringing, diplomatic history points to myriad ways and conditions to accomplish this goal.

NATO membership for Ukraine is not necessarily the outlandish stretch that skeptics claim. Fifteen years ago at its Bucharest Summit, NATO made the decision in principle to welcome Ukraine and Georgia into the alliance, though with the date and conditions left unspecified. It was an awkward compromise forged by NATO’s leaders in real time after they had deadlocked over whether to offer Ukraine and Georgia a preparatory Membership Action Plan (MAP). The result was no MAP or any other way to bring those countries, which had sent forces to NATO missions like Iraq, closer to NATO membership, but a clear statement that NATO membership was the goal.

Agreement on that goal was and remains a big deal: it means, albeit in principle only, that NATO rejects Putin’s assertion that those countries are part of Moscow’s sphere of domination and that if they maintain their systematic democratic transformation, NATO will someday bring them in. That NATO decision, even in the awkward form it was made, infuriated Russian President Vladimir Putin so much that in his speech at the Bucharest NATO-Russia Summit the following day he laid claim to Crimea; to underscore the point, Putin invaded Georgia that August.

NATO retreated from the implications of its own decision at Bucharest: while it has repeated the Bucharest formula at every subsequent NATO summit, it has done so more by rote than conviction.

It’s time to implement the Bucharest decision. NATO membership for Ukraine may be the best way to end the uncertainty about Ukraine’s place in Europe, an ambiguity that Putin has and will continue to exploit. (This would apply to Georgia as well, but that country’s current leadership has been moving in an authoritarian direction and seems to be slipping under Moscow’s influence; Georgia’s leaders do not seem committed to joining the alliance, even in the face of new Russian threats against Georgia and even as public opinion in Georgia continues to vigorously support NATO and EU accession. On March 10, Russian Foreign Minister Sergey Lavrov even extended the threat to “all the countries located around the Russian Federation.”)

Post-Cold War history suggests that NATO accession ends ambiguity and reduces Russian temptation to threaten or pressure; a gray zone is an invitation. NATO membership prevented Russia from doing to Estonia and Latvia what it did to Ukraine starting in 2014: using fabricated claims about alleged persecution of the Russian minority in those countries as an excuse to invade. Making Ukraine a NATO member will anger Russia, but it will bring stability and reduce the threat of repeated Russian campaigns against Ukraine. It’s in the interests of the transatlantic community to deprive Putin of the opportunity to resume his campaign.

Every phase of NATO enlargement has generated opposition, usually centered on concerns over Moscow’s reaction, or defensibility of new members, or burden sharing. Let’s consider those objections to Ukraine’s NATO membership.

Of course, Moscow will hate it. Is that sufficient cause to keep Ukraine in its current ambiguous and exposed state? By launching a dirty war of aggression, including mass killings of civilians, kidnappings, rape, looting and pillaging, all in the service of extermination of Ukrainian nationhood, Russia has forfeited whatever claim it might have made for special consideration. Launching aggressive wars has consequences.

Ukraine is defensible, as the Ukrainians have shown by defending themselves on their own for over a year, supported by U.S. and European weapons and ammunition, but not troops. U.S. and European NATO members initially had no troops at all in NATO’s Eastern flank countries; they started deploying modest forces on a rotational basis– battalion strength in the Baltics and brigade strength in Poland — only after the initial Russian invasion of Ukraine in 2014. After the full invasion in 2022, those deployments increased, but at brigade strength and they are nowhere near the levels of Cold War deployments in West Germany.

It would be better if Ukraine succeeds in liberating all of its territory; should they do so, NATO accession could lock down that success. If Ukraine does not, NATO accession could nevertheless take place. Extending Article 5’s security commitment to Ukraine would not require extending it to Russian-occupied Ukraine; rather, Article 5 could cover free Ukraine at the time of accession and spelled out in the terms of accession. As when West Germany joined NATO in 1955, there is precedent for divided countries to join the alliance. When all of Germany was free, all of Germany joined NATO, a good model for Ukraine.

Under either scenario, carrying out NATO’s Article 5 obligations for Ukraine probably would require a combination of in-country stationing of troops, air support, robust backing of Ukrainian forces (who would continue to carry the main burden of defense), and reinforcement. Given the experience of the Russia-Ukraine War so far, such support would be effective. Expensive? Sure, but compare that price to the current burden of helping Ukraine defend itself right now. Deterring the Russians from attacking a NATO-member Ukraine is apt to be less costly than helping them fight a full-scale war, which is what the United States is now doing.

The U.S. burden in helping defend NATO-member Ukraine would be more than shared by Ukraine’s own commitment to defend itself. Other NATO members, especially Poland and NATO’s Eastern tier members and the U.K. would help in substantial ways.

One big obstacle to Ukraine’s NATO accession could be resistance on the part of governments that opposed MAP for Ukraine and Georgia in 2008, Germany and France. But this opposition may not hold. Germany especially is working through the collapse of its long-held strategic assumptions about Russia as a constructive partner and on European security as something to be built with Russia. German officials now speak of building European security against Russia. German opposition to Ukraine’s NATO membership may change, too. If it does, so may France’s.

Historically, each offer of NATO’s enlargement, especially to Poland, Czechia, and Hungary in 1997 and to the Baltics and four more Central European countries in 2002, required a lot of prior work to build public support in the United States and Europe. Ukrainian accession would as well. The first step is to make the case. And that case can be made.

The United States and Europe need to take seriously the consequences of Russia’s war on Ukraine. They cannot simply seek to patch back together the status quo ante. Instead, they need to consider how to lock in changes that will remove or reduce the likelihood of Russia continuing to start wars in Europe. Arming Ukraine following the current phase of the war – a porcupine strategy – is the current favorite option. That’s not a terrible option. But it may not suffice; it risks yielding the initiative to Russia to prepare for renewed war. NATO membership for Ukraine, agreed 15 years ago as the goal in theory, needs to be made the goal in reality.

The NATO Summit in Vilnius is the place to start. A best-case outcome would be to offer Ukraine a Membership Action Plan, the issue around which NATO deadlocked in 2008 at its Bucharest Summit. In any case, the Vilnius Summit needs to lay out a real path for Ukraine to advance toward NATO. The timing of an offer of accession and the conditions under which it is made cannot be dependent on Russian action: if NATO makes an interim settlement of the conflict or even a ceasefire a condition, Russia will make sure it never happens. NATO needs to make decisions at a scale commensurate with the war Russia started for no good reason, responding to aggression with a bold plan to extend security deeper into Europe.

IMAGE: Protestors make a human chain in front of NATO headquarters, demanding NATO membership and weapons for Ukraine on November 27, 2022 in Brussels, Belgium. (Photo by Thierry Monasse/Getty Images)