Leaders of the UK and France have discussed a ‘coalition of the willing’ to safeguard a potential ceasefire in Ukraine. In this paper, IISS researchers assess three options for a reassurance force, also noting the challenges that may be posed to their deployment.

The UK Prime Minister Sir Keir Starmer and French President Emmanuel Macron recently announced their willingness to create a ‘coalition of the willing’ to safeguard a potential ceasefire agreement in Ukraine. A ‘reassurance’ or ‘deterrence’ force is a better frame for this discussion than a ‘peacekeeping force’, given that one of its tasks would be to respond to a potential Russian violation of a ceasefire agreement. Therefore, agreed response options in the event of a significant breach of a ceasefire would bolster the deterrent value of any deployment. Consequently, the force deployed would need to be credible to Moscow and the coalition resolved to act decisively in the event of a breach of the ceasefire.  

The composition of such a force remains unclear, though both leaders have said a US ‘backstop’ is needed. This IISS analysis sets out three options for such a reassurance force and challenges that may be posed to their deployment: 

• A small-scale force with a deployed land component of a brigade of about 10,000 troops, supported by a limited air component and naval assets;
• A medium-scale force based on a land component of a large division, with about 25,000 troops supported by larger air and maritime components; and
• A large-scale force centred on a corps-sized land component of between 60,000 and 100,000 troops, supported by substantial air and maritime elements.

A small-scale force, potentially led by France and the UK under the Combined Joint Expeditionary Force construct, could be implemented and sustained most straightforwardly, requiring fewer assets and with fewer troops deployed. Its deterrence effect on Russian forces would mostly be achieved by its very presence since its capability for high-intensity combat operations would be limited. 

Medium- and large-scale deployments would increase the reassurance and deterrence effect, but deploying the force would take longer. The divisional-sized force should be able to counter a similarly sized Russian incursion and would be much better placed to protect itself than a single brigade. The large-scale corps-level force – while deploying more slowly – should present a yet more credible military deployment.  

However, the longer the European deployment carries on, and the larger the force becomes, the greater the risk that some of Europe’s capability gaps and shortages become exposed, particularly if the US does not offer a backstop. A US contribution would be particularly important for enablers – such as ISR and strategic intelligence support – while Europe’s other gaps in combat engineering, rocket artillery and suppression of enemy air defences would also become apparent. Moreover, force generation over time could prove a challenge for European countries as troop-contributing states would have to manage multiple troop rotations. Supporting long-term deployments could also cause stress elsewhere, for instance, on countries deploying forces to other missions in Europe.

Such stresses could be eased if a greater number of states took part, such as NATO ally Canada or some of Europe´s Asia-Pacific partners. More participating nations might complicate command and control arrangements and logistics, but their participation would also have political advantages.

In all three options, extra complications would arise should contributing states decide to impose national caveats on their participation, while agreed rules of engagement among all contributing states would also be necessary, as would agreement on acceptable levels of risk.

Europeans could deploy a reassurance force, but without a US backstop, this would become progressively more difficult the larger the force became. At the same time, there is a clear risk that Russia might challenge the force, including by mounting armed attacks – either conventional, sub-threshold, or both.