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. Yet last summer, intelligence agencies repeatedly warned that the Afghan government would collapse and that military leaders were surrendering to the Taliban, Mr. Schiff said.