NATO Unity Tested as Allies Clash Over Security Guarantees for Ukraine

By Ephraim Agbo 

The conversation at NATO headquarters this week—the virtual gathering of 32 defence chiefs to discuss what kinds of security guarantees the West could offer Ukraine as part of any peace deal—was both new and familiar. New because Western capitals are openly sketching what “keeping the peace” might look like inside Ukrainian territory; familiar because the same political and military fault lines that have shaped policy since 2022 reappeared in force. The meeting was described by alliance officials as a “candid discussion” aimed at identifying options that could make a ceasefire durable without immediately creating a NATO combat mission.

Below I unpack what was discussed, why it matters, what it can and cannot achieve militarily, and how Russia — and the United States — fit into the emerging equation.

What was on the table

Allies convened to explore practical options for security guarantees that might reassure Kyiv and reduce the chance of a future invasion. The term being used in public planning discussions is a “reassurance force” or a multinational “coalition of the willing” that could be stationed in Ukrainian territory after a ceasefire to deter renewed aggression. Importantly, those conversations are happening at the military-planning level to shape options, not yet as a political decision to deploy a NATO mission.

At the same time, a separate but connected effort—co-chaired by the UK and France and described publicly as a “coalition of the willing”—is trying to construct the political architecture for a post-ceasefire security presence, the diplomatic outreach, and the non-NATO legal arrangements it might need. That grouping has been explicit about trying to stitch together European partners willing to put forces on the ground and to secure a role for the United States in enabling credibility.

What Washington has said — and what that means

Perhaps the clearest immediate constraint on the reassurance-force concept is the U.S. red line (as currently stated): President Trump has publicly ruled out sending U.S. ground troops to enforce a peace deal inside Ukraine, while leaving open the possibility of air support and other forms of assistance short of boots on the ground. That stance narrows what a credible deterrent can look like, because U.S. involvement — even short of ground combat — is a major credibility multiplier for European partners.

What “no U.S. ground troops” practically means: Europe will have to supply the bulk of any deployed units, and the political architecture will need to compensate with other capabilities (air cover from allied assets, persistent ISR/intelligence sharing, air-defence batteries, and logistics). The U.S. role could still be decisive if it provides realtime intelligence, targeting data, and strategic air support or basing access — but the absence of U.S. brigades in Ukraine changes the deterrence calculus fundamentally.

Who’s willing — and who’s not

NATO is not monolithic on this. Some countries—especially in Central Europe—are deeply skeptical of deploying troops into Ukraine for fear of escalation, while others have been actively planning options and stand ready to contribute if political decisions are made. Public reporting highlights that countries such as Hungary (and in certain reporting, Slovakia) have expressed reservations or opposition to sending forces to Ukraine; others—including the UK, France, and several Baltic and Nordic states—have explored what a reassurance mission might look like and what capabilities they could commit. That political divergence, which maps onto different threat perceptions and domestic politics, will limit how broad any deployment can be.

What such a force could and could not do

A reassurance or coalition force is primarily political deterrence with a defensive military component — not a substitute for the Ukrainian armed forces. Put simply:

• Deterrence and signalling: Presence of Western troops (even in limited numbers) backed by allied air and intelligence assets raises the political and military costs for Moscow of a renewed offensive. It aims to create a clear red line: attacking the area where Western forces are present would risk direct confrontation with those states.
• Limited combat capability: The force would not be structured to defeat a full-scale Russian campaign; it is intended to stabilize, observe, and deter in a post-ceasefire environment while Ukrainian forces remain the principal defenders.
• Dependence on enablers: Absent large U.S. ground formations, European air cover, missile defence, persistent reconnaissance, and logistics will be essential to make the presence credible.

How Russia is likely to view it

From Moscow’s perspective, any Western military presence in Ukraine — NATO or otherwise — is a red line. Russian officials have already warned of consequences if security guarantees are constructed without Russian buy-in. But here’s the nuance: deterrence need not be perfect to be useful. Even a partially credible multinational presence can shape calculations in Moscow, especially if it’s paired with clear political consequences (sanctions, diplomatic isolation, legal commitments) for renewed aggression. The risk is escalation: if Western forces are attacked, the political cost of response could be very high. Amnesty and caution will thus be baked into any deployment planning.

Historical precedent and legal architecture

There is precedent for integrating states into collective defence frameworks despite unsettled borders: West Germany joined NATO in 1955 in a tense geopolitical climate, under conditions that were politically and legally negotiated to manage risk. The modern debate about security guarantees for Ukraine will similarly need legal clarity: are we creating a standing treaty commitment (Article 5-style) or a more circumscribed, time-limited arrangement with specific rules of engagement? The former would be a seismic shift; the latter is more politically achievable — though also less reassuring to Kyiv.

Three scenarios to watch

1. Ceasefire + limited reassurance force: A multinational, primarily European force with U.S. intelligence and air support — designed to deter but not to fight a renewed Russian invasion. Politically plausible but risk of Russian protest and limited deterrence if U.S. engagement is light.
2. No deployment, heavy external guarantees: No foreign troops inside Ukraine, but robust security guarantees, continued arms and air-defence support, and punitive sanctions for any violations. Lower risk of immediate escalation; higher long-term insecurity for Kyiv.
3. Full NATO mission (unlikely today): Would require a political decision that NATO as a bloc commits to defend Ukrainian territory — effectively Article 5-style guarantees or an equivalent. This is politically and strategically the most escalatory and is, for now, off the table.

Bottom line

What allies discussed at NATO this week is real planning for a messy but plausible future: how to turn a ceasefire into a durable settlement without sparking a new phase of escalation. The plan being discussed is as much political theatre as it is military engineering—intended to buy Ukraine comfort that a deal won’t mean a second invasion, while stopping short of the kind of alliance commitment that could trigger a larger war. The credibility gap — especially around U.S. ground forces — is the single biggest constraint. How Europe, the UK, and the coalition of the willing choose to close that gap (with capabilities, basing, and legal clarity) will determine whether reassurance becomes deterrence — or simply a paper promise.