'Longest peacekeeping line in history': The problems with enforcing peace - and why troops could get dragged into fighting
So far, there have only been a few European countries that have offered to put troops on the ground in Ukraine as part of a potential peacekeeping force to protect a deal to end the war.
Michael Clarke begins explaining that peacekeeping forces, in a traditional sense, involve troops "standing on a border, observing what is happening and being prepared to hold the line".
But he says there are two main issues with a peacekeeping force acting in Ukraine.
Two problems - distance and numbers
Firstly, he says it would have to be the "longest peacekeeping line in history by a long way", with the Ukraine-Russia border stretching around 700 miles.
"There's no peacekeeping line in the world that's that long," Clarke says.
"You need however many troops to police it, and most estimates are in the region of 100,000-110,000. You need three times that number available, because for every unit that's there, you need a unit preparing to go in."
He says the alternative is a force of "maybe 50-60,000" placed centrally with lots of mobility across four centres that can reach points of breakdown quickly.
"You need 300,000 troops available from somewhere, but where could the somewhere be?" Clarke asks, adding "that's not Europe".
US void could lead to peacekeeping soldiers having to fight
Clarke says there's also another issue.
While troops from India, Indonesia and South America could technically help make up the numbers required, "they would probably end up fighting".
"Any force that tries to back up this thing without the Americans will find itself involved in shooting," he says.
"And that could go very wrong. If the Americans are there behind it, they might not have to.
"But if the Americans are not behind it, they probably will have to."
Do peacekeeping forces usually work in dangerous areas?
When asked how well peacekeeping forces holding the lines in hot conflicts actually do, Clarke says "variably".
"They did quite well in some parts of the Bosnia crisis.
"But remember Srebrenica, when 8,000 men were marched off to be massacred while the Dutch peacekeepers - who stood there and let it happen because they didn't have any orders - they weren't sure what they should do.
"So if we try and stop this, they'll be shooting. There's lots of civilians. And so they stood there thinking that we don't have any authority to do anything else and surely they wouldn't massacre all these men and boys.
"Well, they did, and the Dutch have been beating themselves up... ever since."
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