Europe’s Moment of Truth on Defending Ukraine—and the Continent

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Paul B. Stares is the General John W. Vessey senior fellow for conflict prevention at the Council on Foreign Relations, where he directs the Center on Preventive Action. Michael O’Hanlon holds the Phil H. Knight chair in defense and strategy at the Brookings Institution. 

While it is still unclear what kind of peace deal in Ukraine the Donald Trump administration intends to reach with Russia, the Europeans should have no doubt as to what will be expected of them afterwards. As National Security Advisor Michael Waltz recently warned: our “underlying principle” is that the Europeans “have to own this conflict going forward. President Trump is going to end it, and then in terms of security guarantees, that is squarely going to be with the Europeans.”

How the Europeans respond in the coming weeks will not only have great bearing on the fate of Ukraine but also on the future of the transatlantic partnership that has been so vital to their security for close to eight decades. Clearly, they have to step up and take more responsibility for their own defense and the maintenance of peace on the continent. If they don’t, the partnership will fray irreparably, and their aspirations to be taken seriously in the emerging multipolar world order will be revealed as empty.

At the same time, though, the Trump administration cannot afford to see the Europeans fail. It must ensure that any cease-fire in Ukraine stays intact and that Russia does not use it simply to pause before resuming its barbaric war of aggression. This requires that the United States continue to provide military assistance to Ukraine and signal that it will backstop European efforts to secure peace. Only through such credible assurances will Russian President Vladimir Putin accept and respect a cease-fire.

To paraphrase the old saw about North American Treaty Organization’s (NATO) founding purpose, the goal for Ukraine should be “to keep the Russians out, the Europeans in, and the Americans on call.” How is this to be done?

The Cost of Defending Ukraine

First, the Europeans must provide the Trump administration with a clear and realistically resourced plan for how they can, together, support Ukraine’s national defense forces in an equitable way for the foreseeable future. We have calculated how Ukraine can provide the core of its own defense requirements with the manpower it has for approximately $20–40 billion a year—not unlike what Israel or South Korea spend for their security. This requires, among other things, continued Western arms supplies, intelligence support, and training. 

Second, the Europeans need to help backstop Ukraine’s security with their own forces. Given Putin’s proclivities, pro forma support for an international monitoring force, whether under the auspices of the United Nations or the Organization for Security and Cooperation in Europe, made up primarily of other nations’ troops is not enough. The Europeans need “to put skin in the game” something that only the presence of a sizable military deployment of about twenty thousand or more troops on Ukrainian soil can do. 

But what organization or coalition could spearhead the effort? One option would be to invoke the “Berlin-plus” formula designed to facilitate the European Union (EU) to draw on NATO assets to support its own Common Security and Defense Policy missions abroad. Going this route would support the EU’s offer that Ukraine become a member of the Union and also signal its intent to be taken seriously as a geopolitical actor. This has been used on at least two prior occasions for peacekeeping operations in the Balkans. Such an approach could also allow the United Kingdom (UK), no longer part of the EU but one of Europe’s top three military powers, to participate and contribute.

A European ‘Coalition of the Willing’

Alternatively, if this option is too difficult because it will require the unanimous support of all EU members—something that cannot be assumed—then the Europeans can organize a coalition of the willing as the next best solution. There’s precedent for such a coalition; the British-led multinational Joint Expeditionary Force (JEF) established in 2018 draws on NATO assets to respond to threats in northern Europe and the Baltic region. But, instead of the JEF’s relatively small rapid reaction force of around ten thousand troops, an EU-led coalition would require something more robust, involving contributions from Bulgaria, France, Germany, Poland, Romania, and the UK.

Militarily, the key idea is this: whatever foreign troops are in Ukraine under a future armistice must be capable of fighting in place to defend themselves against a possible future Russian attack until reinforcements could arrive. If Ukraine does its part and builds a viable self-defense force, the European capability will function primarily as a backup and will not be in acute danger of being overrun in the opening days of a hypothetical war. It should have time to consolidate its disparate elements within Ukraine and organize a serious defense in conjunction with Kyiv. Doing so would not require the two hundred thousand European troops that Ukrainian President Volodymyr Zelenskyy has proposed—but would require perhaps twenty thousand, given how modern militaries are organized and structured. For example, modern brigade combat teams—the key fighting units of Western militaries today—typically consist of 3,500 troops, plus two to three times as many uniformed individuals in support and several thousand more personnel to provide airpower capabilities.

American Security Guarantees

Third, as Europe demonstrates its preparedness to step up, the United States cannot step back. The Trump administration must convey to the Europeans that it stands behind their commitment to Ukraine’s defense—materially and politically—and thus provide the other key element of a serious deterrence strategy against Russia. The Europeans simply can’t do this alone and shouldn’t have to try. The future U.S. military posture in Europe needs to move a bit further east than it was before 2022, with response forces in Poland and perhaps the Baltic states as well. Again, the right ballpark figure is perhaps another ten thousand U.S. troops in Poland or points east, above and beyond the five thousand  troops deployed there in 2021. Those kinds of numbers convey seriousness in the form of real combat power and logistics support. Moreover, the United States should not a priori rule out deploying forces inside Ukraine, something that U.S. Vice President JD Vance has recently indicated is not off the table.

Waltz’s démarche to NATO allies is right—up to a point. The Europeans do need to do more, but two immensely costly world wars in the last century should remind Waltz that this is not the time for the United States to risk a third by hastily disengaging from Ukraine once the guns fall silent.

This work represents the views and opinions solely of the authors. The Council on Foreign Relations is an independent, nonpartisan membership organization, think tank, and publisher, and takes no institutional positions on matters of policy.