Military experts told us who they think would win if Russia invaded Ukraine

• We asked military experts to assess the strengths of Ukraine and Russia should Russia invade.
• Ukraine has been fighting in Donbass for seven years. Its skills and equipment are vastly improved.
• The wild card is the level of NATO and US support for Ukraine, experts said.

Russia this week began huge military exercises in Crimea and along the border with Ukraine, using more than 100,000 troops - backed by heavy armor, airpower, and naval units in the Black Sea - in what many feared was a prelude to an invasion of Ukraine. Then Russian President Vladimir Putin announced he would pull back those troops.

No one knows what his next move will be. But if Russia were to invade, how likely would it be that it could "win"? We asked some experts who have studied both militaries for an assessment.

Russia's military, like its population, is more than triple the size of Ukraine's. But should Putin order an invasion, he would not find it as easy as in 2014, when Russian special forces and local irregular militias seized control of Crimea and much of Ukraine's border with Russia without much resistance.

The reason? Ukraine now has seven years of experience fighting in the Russian-speaking breakaway republic of Donbass, with losses of nearly 10,000 troops in that period. It has also increased its spending and the modernization of its forces with the help of the US and NATO. Donbass was formally part of Ukraine until 2014.

"Ukraine lost a lot of people fighting in Donbass and have learned a lot in that time period," said a NATO military intelligence officer who agreed to assess each side's capabilities in exchange for anonymity.

"They're brave and very patriotic. Ukrainians have no lack of motivation to fight Russians, and they've been fighting along that front for seven years," the NATO official said. "They have very much improved every aspect of what was in 2014 a rotten organization and can really support troops in the field. There's no more sneakers and homemade ammo pouches."

In 2014, Ukraine's underfunded and obsolete Soviet-era military was essentially unable to respond to the events in Donbass and Crimea, leaving Ukrainians to form self-styled and often locally supported militias. While several remain active on the front lines, Ukraine's military is now capable of holding its positions and would inflict heavy casualties on a Russian attack, said a former UK special forces soldier who has spent considerable time in Donbass providing security for media organizations.

'There's no level of digging in that can compensate for the Russian advantage in electronic warfare, cruise missiles, and airpower'

"The Ukrainians are dug in deep and have their artillery dug in and properly sighted," said the former soldier, who declined to be named.

"They're very brave lads, and after this many years they have experienced NCOs," or noncommissioned officers, the former soldier said. "They'd hurt the Russians very badly if not for all the other problems behind the front lines."

Ukraine now spends about $5 billion a year on its military, almost double what it did before 2014. But Russian spends over $65 billion and hosts a much larger and more modern series of capabilities that would make long-term resistance nearly impossible. Its capabilities range from modern air-force and naval assets to supersonic cruise missiles which can accurately hit targets from a safe distance - and which Ukraine lacks the capability to resist.

"As stout as they might be, there's no level of digging in that can compensate for the Russian advantage in electronic warfare, cruise missiles, and airpower," the former soldier said. "So they will be very brave and kill a lot of Russians, but over the medium to long term they have no chance unless NATO helps."

Russia would be able to easily target key infrastructure and facilities far behind Ukrainian lines in the early days of the war. That would eventually reduce the military's ability to fight on the front lines, according to analysts.

"But if NATO can limit the systems the Russians are able to use, then this could become very slow and very nasty for both sides," the former soldier said.

What would Biden do?

The Biden administration is fast-tracking additional military aid to Ukraine, which has requested that the Americans relocate a battery of advanced Patriot missiles from Poland to Ukraine. That move would likely infuriate Putin.

On top of the protection that Patriots could provide from an air attack, the Americans could authorize the deployment of an advanced weapons system already in Ukrainian hands, the Javelin anti-tank missile. Javelins are high-tech weapons designed specifically to fight advanced Russian battle tanks.

These military factors will affect both Biden's and Putin's thinking in the coming days of the continuing crisis.

Costs and risks

Russian Defense Minister Sergey Shoygu said that the troop buildup on Ukraine's eastern flank "demonstrated their ability to provide a credible defence for the country" but that they would return to their regular bases by May 1.

Ukrainian President Volodymyr Zelensky cautiously welcomed the development.

A source previously told Insider that "it wouldn't make a lot of sense for Putin to invade Ukraine." Perhaps the withdrawal announcement shows that Putin agrees - the cost and risks would be high even if Russia succeeded.

Or perhaps that's just what Putin wants Ukraine to think, for now.