Ukrainian citizens learned to make Molotov cocktails from government public service announcements, then recorded themselves setting Russian armored vehicles on fire. Ukraine’s soldiers waited in ambush and fired Western-provided missiles at Russian tanks. The country’s president recorded messages from the streets of his capital, urging his country to fight back against the invaders.

It was a stark contrast from a different set of images, just seven months ago, when the Taliban rolled into Afghanistan’s capital, Kabul, unopposed. Most Afghan troops abandoned their uniforms and weapons. The president fled to the United Arab Emirates, leaving his country to the Taliban militants it had fought for some two decades.

The intelligence community and American military appear to have misjudged both countries’ will to fight, according to lawmakers. In Afghanistan, intelligence agencies had predicted the government and its forces could hold on for at least six months after the U.S. withdrawal. In Ukraine, intelligence officials thought the Russian army would take Kyiv, Ukraine’s capital, in two days. Both estimates proved wrong.

Assessing how well and how fiercely a military, and a nation, will defend itself is extraordinarily difficult. There are many factors to consider, including its leadership, the supplies at its disposal, the strength of the enemy and whether an opposing force is seen as an invader.

The miscalculations demonstrate that even in an age of electronic intercepts and analysis assisted by vast data collection, human relationships still matter in accurately assessing the morale of a country or military. Former intelligence officials say that is why it is critically important that the perspectives of people working directly with partner forces reach policymakers in Washington.

Had the U.S. view of Afghanistan been more realistic, efforts to evacuate Afghans who had assisted the American war effort could have begun earlier — or perhaps some of the billions of dollars put toward training Kabul’s military could have been spent in other ways.

With Ukraine, according to lawmakers on both sides of the aisle, if the United States had had a better sense of how strong and effective the Ukrainian resistance would be against a Russian invasion, it might have sent more weapons to the country sooner.

“Assessing the will to fight in advance of a conflict like this is difficult. However the lesson of the last year is we have to figure out how to do that,” said Senator Angus King, a Maine independent and a member of the Senate Intelligence Committee. “If we had known in advance how strong the Ukrainians would be and how weak the Russians would be, we might have been able to preposition more equipment and had aid to the Ukrainians flow in faster, based on the assumption they had a real chance.”

How badly the intelligence agencies got it wrong is subject to debate. Ahead of the invasion, Ukraine experts “clearly and repeatedly” told policymakers in the White House and Congress that Ukraine’s government and people “probably would resist a Russian invasion,” a U.S. official said.

But intelligence reports are usually hedged. And under questioning from Senator Tom Cotton, Republican of Arkansas, Lt. Gen. Scott D. Berrier, the director of the Defense Intelligence Agency, said this month that, before the invasion, he had thought the Ukrainians were not as ready for an attack as they needed to be.

“Therefore, I questioned their will to fight. That was a bad assessment on my part because they have fought bravely and honorably,” General Berrier said.

In an interview, Mr. Cotton said the intelligence agencies were at their best assessing Russia in the lead-up to the invasion. Once the invasion began, the assessments of Ukraine’s capabilities and Russia’s military were “less than stellar.” Still, he said, judging how effective a country’s defenses will be ahead of a potential attack is tricky.

Will to fight is not a discrete area of intelligence you can go out and collect on it,” Mr. Cotton said. “It’s not like how many working fighters did an air force have? There’s a lot of subjectivity.”

Recent counteroffensives by the Ukrainian military suggest that the country’s leaders are resolved to do more than simply defend Ukraine against the Russian invasion. Over the last week, Ukrainian forces have used tanks and fighter jets to attack Russian positions outside Kyiv and other cities in a way that demonstrates that their objective is not to take back territory, but to destroy Russian forces. It is a sign of not only savvy strategy but a clear intent by Ukraine to defeat the Russian military and win the war.

Representative Adam B. Schiff, Democrat of California and the chairman of the House Intelligence Committee, said it was easy to overstate what the intelligence community got wrong, both in Ukraine and Afghanistan. Last summer, intelligence agencies repeatedly warned that the Afghan government would collapse and that military leaders were surrendering to the Taliban, Mr. Schiff said.

Mr. Schiff said that he had asked during intelligence briefings if Ukraine would fight a Russian invasion and was told by officials that, yes, they would but that it was difficult to know what that would mean in concrete terms.

“If there was a blind spot, I think it was less in believing Ukrainians wouldn’t fight and more about believing the Russian military was more capable than they turned out to be,” Mr. Schiff said.

Russia believed that it would face little effective resistance from the Ukrainian military, and that it could quickly march to Kyiv, rather than having to engage in a slow grinding war, according to U.S. intelligence agencies.

That misjudgment was amplified by the Russian military’s struggle with complex maneuver warfare, supply problems, broken-down vehicles and lack of secure communications, former U.S. intelligence officials said.

“No one doubted the will of the Ukrainians, but given the small size of their army, analysts assessed there were limits to their capability to fight a war on a modern battlefield,” said Douglas H. Wise, a retired senior C.I.A. officer and a former deputy director of the Defense Intelligence Agency. “With the scale of the Russian military dwarfing Ukraine’s much smaller size, analysts ran the numbers and assessed they would not prevail.”

Intelligence officials also had no way of predicting the leadership abilities of Ukraine’s president, Volodymyr Zelensky, which have proven key in rallying the country to the fight. One reason for the misjudgment was that the Ukrainian government, including Mr. Zelensky, was initially skeptical of American intelligence that Russia was going to invade.

Two weeks before the invasion, Mr. King asked intelligence officials how Mr. Zelensky would handle the attack. Mr. King had argued that had President Ashraf Ghani of Afghanistan not fled in the face of an advancing enemy force, Kabul might have lasted longer, and he wanted to know what Mr. Zelensky would do.

“Will he be Churchill or Ghani?” Mr. King asked.

The officials replied that Mr. Zelensky had publicly played down the likelihood of an invasion, but they simply did not know how he would respond.

“But boy, when the chips were down,” Mr. King said in an interview this week, “he channeled his inner Churchill.”

The United States has a bad track record of assessing its partner forces stretching back to Vietnam, when U.S. officials thought the South Vietnamese army would be able to hold off the north after the American withdrawal. Indeed, the more the United States has invested in training partner forces, the less cleareyed officials have been on their prowess.

In Vietnam, Iraq and Afghanistan, American officials believed the units they had trained would fight longer and harder than they did. It is nearly impossible to make an objective analysis of the fighting spirit of a partner force in those situations, former intelligence officers said.

“To get the data you have to become close to your partner and the minute you do that, lack of bias goes out the window,” Mr. Wise said.

Other former intelligence officials argue it is often the officers who train and work with partner forces who can accurately assess the will to fight. But that information is sometimes overlooked as it is passed up to analysts in Washington, said Marc E. Polymeropoulos, a former senior C.I.A. official who oversaw operations in Europe and the Middle East and served several times in Afghanistan.

“If you ask operations officers about will to fight, they will tell you the truth based on their being on the ground with a partner,” he said. “I think any operations officer would have told you that the Afghan regular army did not have that will to fight on their own, if we left, and consistently would have said that over and over again.”

Whether the United States is prepared to handle such assessments better in the future is unclear. It is already confronted with a similar issue as it tries to help Taiwan deter a possible attack from China.

“The question is whether the Taiwanese would exhibit the same fighting spirit as the Ukrainians,” said Rep. Mike Gallagher, Republican of Wisconsin and a member of the House Intelligence Committee. “The answer matters. Because if we suspect not, then we need to be moving more aggressively to help the Taiwan military reform and fix their reservists, the infrastructure and make them a more lethal and more difficult target before it’s too late.”

Intelligence officials believe the Russian war in Ukraine is failing. But they think President Vladimir V. Putin will adjust his tactics, doubling down on the hard-line attacks he has employed in recent weeks or looking to escalate the situation in a bid to force the West to end its support for Ukraine.

The idea that Ukraine is certain to lose may no longer be universally accepted, but some lawmakers think the Biden administration is still underestimating the Ukrainian military.

“Zelensky’s endgame may be victory, it may be getting Russian troops off his soil,” Mr. Cotton said. “Even if you didn’t think that a month ago, you have to concede it is certainly a possibility now.”