Ukrainians fending off Russian attacks will be at an advantage if and when enemy forces arrive in Kyiv, in what could be a bloody battle for the capital, according to an urban warfare expert. 

In addition to the army, Ukrainian civilians have taken up arms in the midst of Russian military aggression that has so far proved difficult for Moscow to confront while still struggling to achieve air dominance. They don't need to win the battles as much as they need to defend themselves, said John Spencer, a retired Army major who saw close combat in Iraq and serves as a colonel with the California National Guard and the director of Urban Warfare Training for the 40th Infantry Division.

"It doesn't have to defeat Russia to win. It just has to not lose. The goal here is just to hold on to what you've got," he told Fox News. "You know Ukrainians are willing to pay the ultimate price. They've already shown that. Russia's not willing to pay the ultimate price to take Ukraine."

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Currently, a Russian convoy 40 miles along is making its way toward Kyiv and Moscow has stepped up airstrikes as it targets large population centers like Kharkiv, Ukraine's second-largest city located in the northeast. Part of Russia's doctrine, Spencer said, was to unleash large amounts of firepower in an effort to create a psychological impact and protect its forces. 

As the slow convoy moves closer to the capital, more attacks should be expected as Russia attempts to "soften" enemy targets and further isolate the city. 

"You can clearly see their plan is to isolate all the eastern Ukraine armies on the east and to further isolate the capital," Spencer said. "That 40-mile convoy's sole purpose is to isolate and penetrate the capital."

If Russia surrounds the city, it could cut off Ukrainian fighters inside, forcing them to hold out for extended periods without reinforcements and supplies from the West as they battle block-by-block, Spencer said. However, an assault on a city versus an open area requires several times more troops than in any other environment, Spencer said.

He noted it usually takes five attackers for every defender, citing U.S. Army doctrine. Because of the terrain, cities are also good for strengthening defense plans.

Despite being outgunned, the Ukrainians can be effective in dense, urban terrain if they use their firepower and surveillance in smart ways, he said.

"The amount of combat power and forces you need to do an operation like this, Russia doesn't have it in its army if the Ukrainians defend right," Spencer said.  

Since the invasion began, videos and news reports have shown Russian forces attacked, their equipment destroyed and some soldiers taken prisoner. The convoy headed to Kyiv is most likely tasked with taking the capital in what could be a large-scale assault, Spencer said. 

In a Twitter thread that went viral over the weekend, he gave a crash course to civilian resistors on a number of tactics to defend themselves, including building obstacles and barriers in the streets and fighting in places where they can take cover instead of exposing themselves in open areas, something urban terrain offer in abundance. 

Part of Ukraine's success is their determination but also because of Russia's failures, he said. 

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"The Russians made so many bad decisions," he said. "It was straight hubris to believe they could take Ukraine in a couple of days."