Britain and France are to lead a military exercise to test a multinational force's ability to deploy to Ukraine, if there is a peace deal.
Nearly 3,000 military personnel from 12 European nations will take part in the rehearsal in neighbouring Poland this month.
Plans for a reassurance force for Ukraine were set out last year as one of the long-term security guarantees being proposed by European nations.
But there are still more questions than answers as to how such a military force might work in practice. Western officials stress that any force sent to Ukraine would not take part in offensive operations or be involved in peacekeeping or ceasefire monitoring missions.
Nor would they be operating under the command of Nato or on the front line.
Officials say their presence would be to provide reassurance to Ukraine and to help rebuild its armed forces.
The US has been involved in discussions, but is not contributing to the mission.
Russia has already voiced opposition to foreign troops being stationed in Ukraine and, last year, President Vladimir Putin said they would be "legitimate targets".
Western officials have declined to spell out the rules of engagement if any international troops were to be attacked in Ukraine. Though they say the force would be able to defend itself.
UK Defence Secretary Wes Streeting said the exercise showed Britain, France and their allies were stepping up preparation for the day when peace came to Ukraine.
He said: "This is not a hypothetical plan, it's a real force, ready to help Ukraine once the fighting stops."
Lt Gen Tom Bateman, the UK commander of the Multi National Force – Ukraine (MNF-U), said the exercise represented an important step in translating political commitments into credible military readiness.
He said it was a "real, capable, readily deployable force that represented a serious commitment from Coalition of the Willing nations to Ukraine's long-term stability and security".
Exercise Koalicja - which takes place from 9-22 November - will be a rehearsal of how to move, protect and supply a force in Ukraine. It will also be a test of their command and control to ensure troops from different countries can work together.
Some 2,800 military personnel will take part across land, sea and air, with the UK contribution including 300 troops and a pair of RAF Typhoons.
Fighter jets will simulate how they would reassure commercial aircraft once Ukraine's skies can be reopened.
Turkey has agreed to lead the maritime mission, with the eventual goal to clear mines and allow the resumption of trade in the Black Sea.
As well as the UK, troops will come from France, Belgium, Estonia, France, Germany, Lithuania, the Netherlands, Poland, Spain, Sweden, and Turkey.
However, Western officials could not comment on the final make-up and shape of the force that could be deployed to Ukraine if the fighting stops.
In the past, military experts have suggested that Western nations might have to send tens of thousands of troops to Ukraine to guarantee its security.
Officials acknowledge that the countries' commitment and contributions will depend on the circumstances of a cessation of hostilities and individual nations' political decisions.
It is also unclear what training role Western forces could provide to battle hardened Ukraine.
Ukrainian soldiers are already in high demand training Nato forces - passing on their expertise in drone warfare.
Above all, Russia has shown no sign of wanting to end its war.
