(NewsNation) — Stalled convoys, reports of fuel shortages, photos of armored vehicles stuck in mud — it’s been nearly three weeks since Russian forces invaded Ukraine and Russian President Vladimir Putin’s initial war strategy appears to have failed.

Despite significant manpower and equipment advantages, Putin’s army has not overrun a much smaller Ukrainian defense force. The capital of Kyiv remains in Ukraine’s control, as does the strategic port city of Odesa, in the southwest part of the country.

At times, Russian forces have appeared overstretched and inept. Pentagon officials have reported that Putin’s army is struggling to feed its troops and fuel its vehicles. Morale is low.

So why was the Russian army so unprepared?

“They made the two biggest mistakes you can make in warfare — they underestimated their opponent and they overestimated their own abilities,” said Dan Emmett, a military expert and former U.S. Marine.

Emmett said the resolve of the Ukrainians, coupled with the Russians’ lack of fighting experience and unfamiliarity with the terrain, has made it especially difficult for the invaders.

putin’s miscalculation

Experts believe Putin expected to capture the Ukrainian capital Kyiv in a matter of days. If he had done so, the Russian president could have ousted Zelenskyy and installed his own pro-Russia government. But that didn’t happen. Instead, Russian troops met stiff Ukrainian resistance and Putin’s soldiers, who were supplied for a short campaign, suddenly found themselves underprepared.

“Putin’s military plan to quickly capture Ukraine, it is clear now, has failed,” Ned Price, State Department spokesman, told reporters on Thursday.

The Russian army’s missteps have played out in videos circulated on social media around the world — armored vehicles bogged down in mud and entire tank convoys ambushed by Ukrainian troops.

For days, satellite images showed a massive Russian military convey stretching 40 miles long sitting stalled outside Kyiv. Once considered among the most powerful fighting forces in the world, the Russian army now appears vulnerable.

That perceived weakness on a global stage has some worried that Putin will escalate the conflict to overcompensate for his army’s lack of progress.

“It looks to me like they’re probably going to hit the cities much, much harder and then only go in with troops after they have destroyed much of the cities,” retired Army Lt. Col. Daniel Davis told NewsNation. “I do fear it could get a lot worse.”

Davis said Russia does not have enough troops to attack each major city one-by-one. Instead, he expects Putin’s forces to use missiles to bombard population centers, given their significant combat power.

Other sectors of Russia’s military have also underperformed. Once thought to be a sure thing, the Russian air force has not established air superiority over Ukraine. That means air support for ground forces has been limited, another contributing factor to the slowed advance.

ukrainian resistance

Above all, experts think Putin greatly underestimated the strength of his opponent. Led by a determined president, Ukrainians of all ages have rallied together to defend their country.

“When you push somebody into a corner and take their home, try and take their cities, they’re going to fight,” said Emmett.

In addition to miscalculating the Ukrainians’ resolve, Emmett thinks Putin overestimated the capabilities of his own troops, many of whom were not battle tested.

“Quite frankly, the Russian army is not that good. It’s made up primarily of conscripts. Some of them don’t even speak Russian. A lot of them don’t even know why they’re in (Ukraine),” he said.

Despite their slow start, Russian forces have advanced in recent days, gaining ground in the push toward Kyiv. But even if Putin’s army reorganizes and defeats Ukraine with more brutal war tactics, Emmett said the resistance inside Ukraine would likely continue, possibly until the invaders leave.

“You’re going to get into phase two, you’re going to get guerrilla warfare, insurgency warfare,” said Emmett.