As Russia and the United States negotiate an end to the war in Ukraine, European leaders are debating how to ensure that any ceasefire leads to lasting peace.
However, military analysts tell the BBC that peacekeeping missions could pose a greater challenge than anything European militaries have faced in the last 30 years.
After Donald Trump announced that he planned to meet Vladimir Putin - "probably in Saudi Arabia" - following a 90-minute phone call last week, concerns grew in Kiev and its European allies that a deal struck without their participation could leave the door open to renewed fighting or future conflicts across the continent.
Britain is "ready and willing" to send troops to Ukraine to guarantee its security as part of a peace deal, Prime Minister Keir Starmer announced on Sunday, reviving an idea first mooted a year ago by French President Emmanuel Macron.
However, after a hastily called meeting with European counterparts in Paris on Monday, where disagreements over the proposal itself reportedly surfaced, Sir Keir stressed that any peace deal would require "American support" to deter Russia from launching new attacks on Ukraine.
The US has ruled out sending its own troops and stressed that the responsibility for guaranteeing Ukrainian security lies with Europe.
This raised doubts about whether the peacekeeping mission could be sustainable without American participation.
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"I wonder where all these soldiers will come from."
"The UK can send and maintain one brigade, which could be around 5.000 people," Dr Frank Ledwidge, a former British military intelligence officer who participated in NATO peacekeeping missions in Bosnia and Herzegovina and Kosovo, told BBC News in Russian.
"In Bosnia, which is about a tenth the size of Ukraine, we had 60.000 peacekeepers, including a large contingent of Americans and a large contingent of heavily armed British."
"The British army is halved in size compared to then and much less armed," added Dr. Ledwidge.
The size of the peacekeeping force and, crucially, the protection of the United States, have guaranteed the success of missions in the past, he explained.
"In all our peacekeeping missions, they were afraid of us, and we had nothing to fear."
"We had guarantees that the Americans had our backs," recalls Dr. Ledwidge, speaking of his service in the former Yugoslavia.
"This would be a much more hostile and dangerous environment than anything we dealt with in Bosnia, Kosovo, and I suspect even Iraq or Afghanistan," he warned.
"We should not send troops until we have solid rules of engagement such as protection from drones and unless the numbers sent are sufficient to keep our troops safe."
Despite these concerns, some see the UK's willingness to take action as a catalyst for wider European involvement.
John Forman, a former military attaché at the British embassy in Moscow, believes that the British leadership could encourage others to follow in their footsteps.
"The UK could probably send 10.000 troops to Ukraine. We need to commit to that so that weaker European countries can participate," he said.
Besides Great Britain and France, it remains unclear which other European NATO members would contribute to the peacekeeping force.
Speaking after the Paris summit, German Chancellor Olaf Scholz said it was "extremely premature and completely wrong to have this discussion now".
But Scholz's reluctance about the timing could be influenced by the upcoming German elections on Sunday, as well as the uncertain situation surrounding the peace talks.
Turkey, a NATO member with the second-largest military after the US, has yet to declare its own position on the potential mission.
Ukraine's neighbor Poland, which has the third-largest army in NATO, has already ruled out sending its own forces.
Without these key players, analysts estimate that the remaining NATO members can collectively send a maximum of 30.000 troops – far fewer than the hundreds of thousands that Ukrainian President Volodymyr Zelensky has claimed will be needed.
Some argue that securing key parts of the front, with Ukrainian forces playing a central role, could reduce that demand.
Starmer is preparing to visit Washington next week to meet with Trump, where he is expected to seek support for his plan.
Fred Fleitz, a foreign policy adviser close to the US president, told BBC Radio 4 that it was "an incredibly generous offer from your prime minister" and that it would be a "terrific contribution".
Whether Russia would tolerate such a move is another question entirely.
Foreign Minister Sergei Lavrov said sending foreign troops from NATO countries to Ukraine would be "unacceptable," even if they operated under other flags, warning that Moscow would see it as an expansion of the alliance, echoing Putin's justification for a full-scale invasion.
Meanwhile, European countries remain excluded from the peace talks, with Russian and American officials having already met without them.
The key question now is whether Europe has enough leverage to convince the United States that it is necessary to send peacekeepers to Ukraine if a ceasefire is agreed in Saudi Arabia.
For now at least, Russia sees Europe's role as prolonging the war rather than ending it.
Additional reporting: Daniel Wittenberg, from Paris
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