Richard Gowan, a fellow at the European Council on Foreign Relations, reflected on whether a UN-backed peacekeeping force could end the grinding war in eastern Ukraine that has claimed more than 10,000 lives.

"The Russians want something light. Ukraine would like a hefty U.N. force to quickly take control of the breakaway areas of the Donetsk basin, or Donbas — but recognizes that Russian-occupied Crimea is off-limits," Gowan wrote for The Washington Post.

The expert explored the lessons of past operations for a deployment in Donbas.

Such a force would aim to facilitate a cessation of hostilities, oversee local elections that Russia and Ukraine have agreed should be a precondition for a political settlement, and facilitate a transition culminating in the full restoration of Kyiv’s authority.

Gowan has named five criteria for a successful operation:

Read alsoU.S. questions how much it can trust Russia on peacekeepers in Ukraine1) A genuine bargain with Russia: Moscow has orchestrated the war in the Donbas since its outset. No peace operation can succeed unless the Russians give it a chance. Ukraine and its Western partners need to be confident of Russia’s good faith before backing a Donbas mission.

2) A credible military presence: Even with Moscow’s backing, peacekeeping missions could be at risk from militant campaigns. Some analysts argue that containing this threat would take 50,000 troops but such a force could be hard to generate, especially as Russia is likely to object to major NATO nations participating. A more tenable figure might be about 20,000 troops.

The exact numbers are less important than troop capabilities and commitment. The United States has lobbied Sweden to lead a force, and Stockholm has indicated wary interest. It might be necessary to cobble together a wider force from other Nordic states and more-distant options such as Mongolia, whose troops have performed courageously for the United Nations in South Sudan.

Read alsoPutin: Russia "not against" putting occupied Donbas under int'l control3) A strong police presence: A credible military component would also need a serious police complement, as military-only operations tend to deal poorly with civil disorder. In 1999, NATO troops in Kosovo largely stood by as the ethnic Albanian population attacked Serb communities. Such presence might be quite large: The United Nations ultimately deployed more than 3,500 police officers to Kosovo. A Donbas mission could require 2,000 to 4,000 police officers. Some European countries — including Portugal, Italy and Romania — have good histories of “police-keeping.” But Russia would have to overlook their NATO status.

4) A long-term civilian presence: Most peace operations now involve sizable civilian presences (10 to 20 percent of an overall deployment) charged with responsibilities including mission administration and governance reform. In Donbas, the civilian priority would be arranging local elections. While most military peacekeepers would hopefully leave the Donbas after a matter of years, deeper peace-building could take decades.

Read alsoDeputy Minister: "I consider enemies those calling to let go of Donbas5) A supportive Ukraine: The final criterion for a successful mission would be genuine support from Kyiv. Many Ukrainian politicians and commentators want to see the separatists in the Donbas punished — with talk of criminal prosecutions and stripping local officials of their jobs. There is no chance of a stable peace if Ukraine aims for widespread punishment.

Ultimately, all peace operations are acts of political compromise. All sides have to accept second-best solutions to end a war. In the Donbas, a peacekeeping force will make sense only if both Moscow and Kiev want to make it work. If they do, and it is possible to find enough capable units, an operation should just about work. But with the West’s relations with Russia in constant turmoil, the odds are still daunting.