‘By the evening, I think half of Ukraine will be Russian’: Residents scramble to flee Kyiv.

But even with the threat of substantial sanctions and allied unity, it was not enough in the end to deter Mr. Putin from carrying out the broad assault that got underway early on Thursday.

But it improved Washington’s ability to bring the trans-Atlantic alliance into a unified front against Moscow and to prepare waves of sanctions and other steps to impose a cost on Russia. And after high-profile intelligence failures in Afghanistan, Iraq and other global crises over the past several decades, the accuracy of the intelligence and analysis about Mr. Putin gave the C.I.A. and the broader array of U.S. intelligence agencies new credibility at home and abroad.

The result has been a remarkable four months of diplomacy, deterrence and American-led information warfare, including a last-ditch effort to disrupt Mr. Putin’s strategy by plugging into the Russian military’s plans and then exposing them publicly. Unlike the withdrawal from Afghanistan, it was executed almost flawlessly. Even the Germans and other European nations highly dependent on Russian-supplied gas signed onto the playbook.

The U.S. used its intelligence in innovative ways as the crisis built. William J. Burns, the C.I.A. director, confronted the Russian government with its own war plans. Avril D. Haines, the director of national intelligence, shared secret intelligence with allied governments to build support for the American assessment. And the White House and State Department shared some declassified intelligence publicly to expose Mr. Putin’s plans for “false flag” operations and deny him the pretext he wanted to invade.

The intelligence disclosures may not be over now that the invasion has begun. The Biden administration has made clear it does not want to take on the job of publicly calling out Russian troop movements. But the United States is considering continuing its information releases, mulling various options to hold Russia accountable for actions it will take in Ukraine, according to people familiar with the discussion.

Those new efforts could involve countering Russian propaganda that they are guardians and liberators of the Ukrainian people, not an occupying force. They could also involve work to expose potential war crimes and try to give the lie to Russian claims that their war aims are limited.

Mr. Putin’s plan to topple the government in Kyiv was his goal from the beginning, American officials have said, and some officials are keen to show Russia is simply carrying out a plan crafted months ago.

“It’s not something you want to do forever or as a permanent feature of policy or it loses its novelty, but in extraordinary, life-or-death situations, it is justified,” said John E. McLaughlin, a former acting C.I.A. director. “I always found in confronting Russians with our knowledge of what they were doing, that they would inevitably deny it but that it threw them off balance to know that we knew. And I think it has rattled Putin this time.”

In the end it was not enough to stop Mr. Putin, though it is not clear what strategy, if any, he might have.

The American effort to reveal Mr. Putin’s plans to the world, has “been a distraction to him, it’s been somewhat annoying,” James Clapper, the former director of national intelligence, said Wednesday. But, he added, “It remains to be seen what difference it has made on his decision-making.”

Some of information the United States shared with allies, beginning with a trip to NATO by Ms. Haines in November, was initially greeted skeptically, according to Western officials. Many Europeans still remember the bad intelligence around the Iraq war.

But as the information provided grew and the Russian war plan played out as Ms. Haines had predicted, European officials shifted their view. The intelligence-sharing campaign ultimately succeeded in uniting Europe and America on a series of tough sanctions.

Republicans have been critical of Mr. Biden for not being more aggressive in the military supplies it sent to Kyiv or acting earlier to impose stiff sentences on Russia to change Mr. Putin’s course of action.

It will take time to know if more and better weapons could have made a difference for the Ukrainian army’s resistance. But administration officials have said they have had to act judiciously not to escalate the situation and not allow Mr. Putin to use American military supplies as excuse to start the war.

More clearly, American sanctions against Mr. Putin go only so far. It is European sanctions against Russia and its billionaire class that really bite, and it took time, and intelligence, for Europe to come on board with a tough package of sanctions.

While the United States clearly has the some of the best, if not the best, intelligence collection in the world, it also had a reputation that remained tarnished, at home and abroad, by the 2003 Iraq invasion, when faulty information was publicly released to justify the war. While the intelligence community had long been pessimistic about the survival prospects for the U.S.-supported Afghan government, some in the administration criticized the spy agencies last year for not accurately predicting how quickly the country’s military forces would fold.

There is little doubt that reputation increased some of the skepticism of the assessment of Mr. Putin’s intentions, both by reporters questioning public officials for more evidence, and by allies.

The warnings this time were far different, the information released to try to prevent a war, not to start one. But releasing the information was nevertheless a risk. Had it proved wrong, the intelligence agencies would have been saddled with fresh doubts about their ability to collect and properly analyze intelligence about an adversaries’ capabilities and intentions. Their ability to credibly warn against future threats would have diminished.

Instead, the public got a rare glimpse of an intelligence success. It is usually the failures, or partial failures, like Iraq, the Sept. 11 terror attacks, the surveillance of domestic civil rights groups or the Bay of Pigs, that are publicly aired.

But the failures do not mean America’s spy agencies do not have many successes, said Nicholas Dujmovic, a former C.I.A. historian who now teaches at the Catholic University of America.

“This is a rare case that intelligence successes are being made public, and the public should conclude, in my view, that this is rather the norm,” Dr. Dujmovic said. “They are getting a rare glimpse of the normal process and production of intelligence that normally they do not see.”

Most accusations of intelligence failures are failures to properly warn about an attack or to overstate a threat. And it is those warnings that this time proved prescient.

“The warning analysts have the hardest job in analysis because they are trying to figure out intentions — whether the attack will come, when it will come, how it will come,” Dr. Dujmovic said. “The best way to penetrate that fog is with a human source close to the decision maker, in this case, Putin — and it’s also the hardest kind of collection to acquire.”

The intelligence agencies succeeded in divining Mr. Putin’s intentions early on. And that was no easy feat. It is simply not publicly known how strong is America’s source network in Russia or how close those people are to Putin, but it is clear Mr. Putin shares his counsel with very few.

Monday’s televised meeting of Russia’s national security aides showed the foreign intelligence chief being berated by Mr. Putin for failing to endorse recognition of the breakaway enclaves in Eastern Ukraine. Juxtaposed with the months of American disclosures, the scene suggested that people atop America’s spy agencies, for once, may have understood Mr. Putin’s intentions better than his own intelligence officers.