Russian President Vladimir Putin underestimated the strength of the Ukrainian resistance as well as the harsh backlash from the United States and Western allies when he invaded Ukraine last month, experts tell Fox News. 

The U.S. continued piling sanctions on Russia Tuesday, implementing a ban on oil imports from the country in what President Biden called "another powerful blow to Putin's war machine."

"Putin miscalculated the Ukrainians’ willingness to fight, the leadership style and willingness to die for the cause of [Ukrainian] President [Volodymyr] Zelenskyy," Rebekah Koffler, a former Defense Intelligence Agency officer and author of "Putin's Playbook: Russia's Secret Plan to Defeat America," told Fox News Digital. She said he also miscalculated "the backlash of Western audiences, Europeans, Americans and even a segment of the Russian population who are anti-war."

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Zelenskyy, who has already survived three assassination attempts, vowed to stay in the Ukrainian capital of Kyiv as Russian forces lay siege to the city, even going so far as to reveal his location in a defiant message on Monday. 

"On Bankova Street," Zelenskyy said Monday, referring to where presidential offices are located. "Not hiding, and I’m not afraid of anyone."

Russian President Vladimir Putin

Russian President Vladimir Putin has signed a law that could punish journalists with up to 15 years in prison for reporting so-called "fake" news about his military invasion of Ukraine. (Yuri Kochetkov/Pool via AP)

Unarmed Ukrainian civilians have confronted Russian soldiers as thousands of foreigners have traveled to Eastern Europe to help the Ukrainian army defend the country. 

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As the Russian military attempts to take Ukrainian cities, unprecedented sanctions on Russian banks, oligarchs and industries could challenge Putin domestically. 

"The sanctions could make it very difficult to govern Russia, in the sense that people's savings have been wiped out, factories will start to close, and there are fewer high-tech imports that are needed for the Russian economy. And obviously, the financial elite has taken a real beating," Timothy Frye, the professor of post-Soviet foreign policy at Columbia University, told Fox News Digital. 

Ukrainian President Volodymyr Zelenskyy speaks in Kyiv, Ukraine

In this March 8, 2022, image from video provided by the Ukrainian Presidential Press Office and posted on Instagram, Ukrainian President Volodymyr Zelenskyy speaks in Kyiv, Ukraine. (Ukrainian Presidential Press Office via AP)

Companies started pulling out of Russia almost as soon as Putin invaded, both due to Western sanctions and to their disapproval of the war. Food giants Starbucks, Coca-Cola and Pepsi joined the withdrawal on Tuesday. 

"At some point, Putin, who has long been averse to any kind of domestic political instability, might fear the reaction from the elite and from the mass public, and begin to look for a way out of this situation, but we're not there yet," Frye said. "It could take a while before we get there."

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Putin, a former KGB officer known for his ruthless information warfare tactics, also failed to take into account how a war would play out in 2022. 

"[Putin] didn't anticipate how technology has brought this conflict into the homes of ordinary people all over the world, by virtue of it unfolding on our TVs and on our computer screens," Koffler said. "He was counting on the fact that he could keep it hidden, not only from the Russian people, but also from the rest of the world. Well, it's no longer the case." 

Russian authorities have cracked down on dissent over the last two weeks, blocking independent and foreign news outlets while shutting off access to social media platforms like Twitter and Facebook. 

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While Putin still has control over the flow of information at home and it is difficult to get a pulse on public opinion in Russia, Frye said the tide could turn. 

"The message will slowly get through, particularly if the military gets bogged down significantly," Frye said. "This is an extraordinarily volatile time, I think, in Russian public opinion towards the war."\