The Biden administration’s secret planning began in April 2021 when Russia massed about 100,000 troops on the Ukrainian border. The buildup turned out to be a feint, but Blinken and other officials discussed U.S. intelligence about Russia’s actions with leaders of Britain, France and Germany at a NATO meeting in Brussels that month. Their message was, “We need to get ourselves prepared,” a senior State Department official said.

Germany was a reluctant but essential ally, and the Biden administration made a controversial decision last summer that was probably crucial in gaining German support against Russia. Biden gave Germany a pass on an initial round of sanctions against a company building the Nord Stream 2 pipeline in exchange for a pledge from Chancellor Angela Merkel that if Russia invaded, Nord Stream 2 would be scrapped. When the invasion came, Merkel was gone but her successor, Olaf Scholz, kept the promise.

By avoiding a crisis with Germany early on, Blinken said, “the net result was that the foundation was in place when the Russians went ahead with the aggression.”

This U.S. diplomacy gets high marks from Emily Haber, the German ambassador to Washington. “The wording in the joint statement [about Nord Stream] was vague, but the administration trusted the old — and later the new — chancellor to follow up on it. Which is what happened,” she told me. “A sublime form, I thought, of partnership management.”

The Ukraine threat got red-hot in October, when the United States gathered intelligence about a renewed Russian buildup on the border, along with “some detail about what Russian plans for those forces actually were,” Blinken said. This operational detail “was really the eye opener.” The Group of 20 nations were meeting at the end of October in Rome, and Biden pulled aside the leaders of Britain, France and Germany and gave them a detailed readout on the top-secret evidence.

“It was galvanizing enough that there was an agreement … to fleshing out the consequences for Russia if it went ahead with the aggression,” Blinken said.

CIA Director William J. Burns traveled to Moscow on Nov. 1 to warn President Vladimir Putin that the United States and its allies were prepared to arm Ukraine and impose crippling sanctions on Russia if he invaded. Putin apparently thought Biden wouldn’t be able to deliver.

Persuading Ukrainian President Volodymyr Zelensky to take the invasion danger seriously wasn’t easy, initially. Blinken spoke to him at the COP 21 climate summit in Glasgow in early November and provided a summary of intelligence about Russia’s plans. “I basically had the task of telling him that we thought it was likely that his country was going to be invaded,” Blinken recalled. Zelensky was skeptical, according to a State Department official.

Threatening sanctions can be an empty diplomatic ritual. But in December, Blinken and his colleagues began seriously discussing with allies what steps they would take. The initial venue was a Group of Seven foreign ministers meeting in Liverpool, England, on Dec. 11. The attendees publicly committed that there would be “massive consequences and severe costs,” Blinken remembered. As a result, he said, “when the aggression actually happened, we were able to move immediately.”

NATO military planning accelerated along with the diplomacy. Air Force Gen. Tod Wolters, the NATO commander, told me that his colleagues began preparing in December and January the “ground lines of communication” that would allow rapid shipment of arms into Ukraine. They studied entry points for supplies and other practical details. This weapons pipeline delivered Stinger and Javelin missiles before the invasion began Feb. 24 and has transferred huge numbers of heavier weapons since then.

U.S. intelligence provided Ukraine with a preview of Putin’s battle plan. Though Russia had surrounded Ukraine with 150,000 troops, Putin’s real strategy was a lightning, decapitating strike on Kyiv by a relatively small group of elite special forces. The Russians planned to seize Antonov Airport in Hostomel, west of the capital, and then use it to quickly pump troops into Kyiv.

The Ukrainians knew the Russians were coming. Burns had secretly traveled to Kyiv in January to brief Zelensky on the Russian plan, according to two knowledgeable officials. The Ukrainians used the U.S. intelligence to devastate the attacking force at Hostomel, in what may turn out to be the decisive battle of the war. “The Russians had no Plan B,” explained Marek Menkiszak, a Polish intelligence analyst with the Centre for Eastern Studies in Warsaw.

Menkiszak explained the significance of intelligence coup that revealed the decapitation plan: “The Russians trapped themselves. … It was not meant to be a full-scale war but a special operation” that would topple Zelensky’s government and install a pliant, pro-Moscow regime.

Through the buildup to war, Biden sometimes seemed to misspeak. But he had a clear-eyed view of the evolving strategic terrain. Early on, for example, Biden concluded that the best way to derail Putin’s hope for dividing NATO would be the accession of two strong new members, Finland and Sweden.

Biden wooed Finnish President Sauli Niinisto. He called him in December and then in January to talk about the Russian threat, Blinken said. Biden then invited Niinisto to visit the White House in March, and while they were sitting in the Oval Office, Biden suggested they call Swedish Prime Minister Magdalena Andersson, reaching her late at night. By May, the two were visiting the White House together, celebrating their countries’ plans to join NATO.

The Biden administration’s organization of this coalition to support Ukraine may look simple in retrospect. But it was a complicated coordination of diplomatic, military and intelligence resources that pulled together dozens of nations at what may prove to be a hinge point in modern history. Putin thought he could roll through Biden and the West to an easy victory in Kyiv. The Russian leader made a catastrophic mistake in overvaluing his own strength and underestimating the resolve of Biden and his team.