Ukrainian forces have pulled off an extraordinary feat in holding off Russian forces for days, but President Vladimir Putin’s superior manpower and weaponry will likely triumph at a substantial cost to the autocratic leader and his people, experts told Fox News Digital.

"It is nothing short of remarkable what the Ukrainian military has accomplished," said U.S. Army Gen. Jack Keane, who serves as chairman of the Institute for the Study of War.

"But I think despite the fierceness of the Ukrainian resistance, they are overmatched by the Russians, and the Russians will eventually prevail," Keane predicted.

Zelenskyy and Putin

Ukrainian forces, which by some estimates are outnumbered by Russian troops two to one, have stalled the Kremlin’s advance on three of four avenues of approach. 

A Ukrainian marine allegedly detonated explosives on a bridge to thwart Russia tanks from advancing toward Kyiv – even though he knew he could not flee the blast zone in time, according to a statement from the Ukrainian military

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Russian soldiers have streamed into Ukraine from Belarus in the north toward Kyiv, from Crimea to the south toward Odesa, from Russia to the east toward Kharkiv and the Donbas region. The advance from Crimea is the only front where Russia has made progress, Keane said.

Russia has superior manpower

Russia had over 900,000 active military personnel and about 2 million reservists in 2020 compared to Ukraine’s 209,000 active military personnel and 900,000 reservists in the same year, according to the 2021 "Military Balance" report from the International Institute for Strategic Studies. Russia spent about $40 billion or about 4% of its GDP on defense, while Ukraine spent about $4 billion or 3% of GDP in 2020.

Ukraine national flag Kyiv

Ukraine's biggest national flag flies in Kyiv on February 26, 2022.  (GENYA SAVILOV/AFP via Getty Images)

"It is significant that not a single population center has been captured after five nights and four days," Keane said. He added that Russia still does not control the airspace and the supply lines feeding the offensive are running out of fuel, ammunition and water.

"If at some point [Putin] has to reduce Kyiv to rubble and kill thousands of civilians to take control of the government, that is what he will do because that is who he is," said retired U.S. Army Gen. Jack Keane

The longer it takes for Russia to overtake Kyiv, the Ukraine government’s seat of power and the Kremlin’s main target, the more adverse the long-term outcome for Putin, the four-star general told Fox News Digital.

He called Putin’s offensive too "ambitious and complicated." 

Keane said it is clear from Putin’s campaign he is trying to reduce civilian casualties, but as the autocratic leader grows increasingly more frustrated, he may loosen his restraint. "If at some point he has to reduce Kyiv to rubble and kill thousands of civilians to take control of the government, that is what he will do because that is who he is," he said.

Ukrainian President Volodymyr Zelenskiy

Ukrainian President Volodymyr Zelenskyy speaks alongside other Ukrainian officials in the governmental district of Kyiv, confirming that he is still in the capital, in Kyiv, Ukraine February 25, 2022, in this screengrab obtained from a handout video.  (Ukrainian Presidential Press Service/Handout via Reuters)

Putin put down separatist uprisings in Chechnya with grotesque force, leveling cities and killing thousands of innocent civilians. 

In Syria, Putin helped President Bashar Al-Assad retain power amid a civil uprising in 2011 by carpet-bombing whole villages and neighborhoods, killing thousands of innocent people. Russian forces used penetration bombs on underground hospitals, according to Keane. 

Stiff Ukrainian resistance

Retired U.S. Army Lt. Col. Anthony Shaffer, president of the London Center for Policy Research, said it is hard to know exactly what is happening on the ground in Ukraine, with both sides launching propaganda campaigns to achieve a psychological advantage. 

Woman injured in Ukraine after Russia's invasion

A wounded woman is seen after an airstrike damaged an apartment complex in city of Chuhuiv, Kharkiv Oblast, Ukraine on February 24, 2022. (Photo by Wolfgang Schwan/Anadolu Agency via Getty Images)

But, Shaffer said, it is clear that the Ukrainians have been more effective than the Russians had expected.

"The Russians miscalculated the amount of resistance," he said. Putin, he believes, erroneously assumed that Ukraine would surrender the moment troops crossed the border and that President Volodymyr Zelenskyy would flee.

Zelenskyy, instead, has defiantly remained in the city with government leaders and ministers, vowing to fight the incursion with his people, and supplying weapons to anyone willing to fight.

Putin thought he could pull off victory with special operations that would target Kyiv and Zelenskyy.

"He was going for a decapitation of the Ukrainian leadership," Shaffer said. "If you cut off the head you don’t need to invade everywhere else."  

Damage in Ukraine after Russia's invasion

Flame and smoke rise from the debris of a privet house in the aftermath of Russian shelling outside Kyiv, Ukraine, Thursday, Feb. 24, 2022. (AP Photo/Efrem Lukatsky)

Putin only mobilized an estimated 250,000 soldiers— not nearly enough troops for a full-blown offensive against a Ukrainian force of about 110,000, Shaffer.

Shaffer, who ran key intelligence operations against Russia, said arming citizens with weapons and encouraging them to make Molotov cocktails is an effective strategy.

"This is one of the reasons Japan never invaded the U.S., they felt there would be an American with a gun behind every bush," said etired U.S. Army Lt. Col. Athony Shaffer

"The left is always asking why do you need guns. This is why, to have a large population armed and able to defend its terrain," he said. "This is one of the reasons Japan never invaded the U.S., they felt there would be an American with a gun behind every bush."

The Ukrainians are fighting not to lose while the Russians have to actually win, he said. At this point, Putin must redirect troops and substantially increase supplies for a war that is costing him $20 to $25 billion a day.

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Valeriy Dzutsev, a visiting assistant professor at the University of Kansas who specializes in Russia and Ukraine, said it doesn’t look good that Putin has initiated negotiations with Zelenskyy.

"Even if we have no other information, we know this is not good for Russia," he said, calling the Kremlin’s military might "exaggerated." 

He said that Putin is in a tough position because he really can’t use heavy, indiscriminate bombardment without losing domestic support and further taunting the West. "There is a cost to a scorched earth type of warfare," he said. "There are limitations."

A pyrrhic victory

If Putin secures victory, which Keane, described as inevitable, it will be a pyrrhic one, the general said.

"I believe Putin has strategically overreached and will suffer long-term consequences for this," Keane told Fox News Digital.

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The war will likely lead to Russia becoming an international pariah and to a rejuvenation of NATO — a development Putin has fought against for more than two decades. He may lose domestic support as his people suffer under the weight of devastating sanctions and a protracted war of insurgency, according to Keane.

"Ukrainian people will not be defeated and will resist him and fight him for years [even if there is regime change]," Keane said. "Putin doesn’t seem to be thinking clearly. What is his emotional, psychological and mental capacity? People are beginning to wonder."