Russia could invade Ukraine, but Putin might be afraid of the backlash, says ex-U.S. ambassador

Many people believe that Vladimir Putin could invade Ukraine, but the Russian president may be afraid of the consequences that Washington has warned about, a former U.S. ambassador to Ukraine said Monday.

The notion that Russia may escalate the situation in Ukraine is "reasonable" given the thousands of troops at the border and the constant aggression from Moscow, said John Herbst, who is now senior director of the Atlantic Council's Eurasia Center.

The United States and its allies are concerned that Russia will attack Ukraine, possibly in the coming weeks, though Moscow says it has no plans to invade the Eastern European country.

U.S. President Joe Biden's framework to push back against Russia consists of sending weapons to Ukraine, moving NATO forces along Russia's border and punishing sanctions if an invasion happens.

"My personal view is that's a possibility, but I think [Putin] is afraid of the response," Herbst told CNBC's "Street Signs Asia" on Monday.

"If Putin decides that, in fact, the Biden response is more bluff than reality, Russian troops may wind up in large numbers in Ukraine, beyond the numbers that are currently in Donbas and in Crimea," he said. "I suspect that at a minimum, this confrontation will play out for six or eight weeks."

Herbst said the United States and its allies need to push back hard against Russia since appeasement did not work in the past when Moscow invaded Georgia and Crimea and "suffered almost no consequences."

The U.S. plan now is "reasonable," but not sufficiently active, he said. "It needs to be stronger and faster, and we need to do it in conjunction with our allies."

Additionally, Herbst said there's "almost no chance of Russia winning" a long-term war with Ukraine.

That's because a large majority of Ukrainians "despise the Kremlin" for its aggression and will fight back. Moscow's military is much stronger, but in the long run, it will be difficult for Russia to hold on to cities it has seized, he predicted.

Russia's overall geopolitical position will also worsen if it invades Ukraine, he added.

"If Russia sends those troops into Ukraine, the chances of Sweden or Finland joining NATO go way up," he said, adding that Putin's actions have made NATO membership a "serious question" for the two countries.

For now, it's unclear what will happen next, but Putin sees the threat of a military conflict as something that improves Russia's negotiating position, according to Oleg Ignatov, senior analyst at the International Crisis Group.

It's a good sign that Russia hasn't withdrawn from talks even though they have not yielded any results, he said.

"If Russia withdraws, it will mean war," Ignatov told CNBC's "Capital Connection" on Monday. "To continue negotiations is the best solution that is possible right now," he said.