Smoke hung over the gray streets that day in Kyiv, where protesters had piled tires, furniture and barbed wire to barricade themselves from security forces. Torn blue and yellow Ukrainian flags whipped in the wind, and candles left on sidewalks marked where people had been gunned down. A drawing of a reviled president depicted as a pig was tacked to a lamp post.

And yet there was a feeling of hope in Kyiv in March 2014, as Secretary of State John F. Kerry met with survivors of a violent crackdown on demonstrations. He commended the Ukrainians for their bravery in confronting a Kremlin-backed leader and promised that the United States would support the new government.

But Russian forces had moved into Crimea, Ukraine’s peninsula on the Black Sea, and Mr. Kerry warned: “It is clear that Russia has been working hard to create a pretext for being able to invade further.”

Eight years later, with Russian troops obliterating Ukrainian cities and towns, Mr. Kerry’s words seem eerily prescient.

Through the administrations of three American presidents, the United States has sent mixed signals about its commitment to Ukraine. All the while, President Vladimir V. Putin of Russia watched Washington’s moves, biding his time.

“We’ve been all over the place on Ukraine,” said Fiona Hill, a Russia and Eurasia expert who advised the three administrations before President Biden. “Our own frames have shifted over time, and our own policies have shifted.”

“I think we need to re-articulate why Ukraine matters,” she said.

Now, two months into Mr. Putin’s war, the United States is at the center of an extraordinary campaign to foil him, casting the military conflict as a broader battle between democratic values and authoritarian might.

“It’s nothing less than a direct challenge to the rule-based international order established since the end of World War II,” Mr. Biden said in Warsaw last month. “And it threatens to return to decades of war that ravaged Europe before the international rule-based order was put in place. We cannot go back to that.”

The United States has rushed weapons and humanitarian aid to Ukraine and imposed sanctions intended to cut off Russia from global markets. This past weekend, Mr. Biden sent Secretary of State Antony J. Blinken and Defense Secretary Lloyd J. Austin III to Ukraine as affirmation of Washington’s support.

In many ways, officials said, Mr. Biden is trying to make up for the years of U.S. indecisiveness toward Kyiv. Those who wavered earlier include top Biden aides who had worked in the Obama administration as well as officials in the administration of Donald J. Trump, who undermined U.S. policy on Ukraine for personal political gain, according to current and former officials and a review of records.

The Roots of War

Since the earliest days of Ukraine’s independence, in 1991, American officials have recognized the country’s strategic value as Russia struggled to find its footing after the breakup of the Soviet Union.

“Without Ukraine, Russia ceases to be an empire,” Zbigniew Brzezinski, who had been the national security adviser to President Jimmy Carter, wrote in a March 1994 essay. “But with Ukraine suborned and then subordinated, Russia automatically becomes an empire.”

Two months earlier, under pressure from the United States, Ukraine had reached an agreement to destroy its nuclear arsenal. President Bill Clinton heralded the pact as “a hopeful and historic breakthrough” to improve global security. But Ukraine’s leader, President Leonid Kuchma, warned that it would make his fledgling country more vulnerable.

“If tomorrow, Russia goes into Crimea, no one will raise an eyebrow,” he said that year.

At the time, Moscow was already goading a separatist movement in Crimea, even as Mr. Clinton predicted that Ukraine would become a major European power.

Yet over the next decade, experts said, NATO left out Ukraine to avoid angering Russia, which some members saw as an important economic partner and energy supplier and hoped would evolve into a more democratic and less threatening power.

The Baltic States joined NATO in 2004, and four years later, President George W. Bush publicly backed Ukraine’s ambition to follow. But Western European nations were reluctant. Today, Ukraine is neither a NATO member nor a part of the European Union, and officials cautioned as recently as this month that its inclusion in either was far from likely.

Years after Mr. Bush’s show of support, a new Ukrainian president, Viktor F. Yanukovych, tried to move the country closer to Russia, sparking mass protests in November 2013 when he refused to sign a long-planned agreement to strengthen ties with the European Union.

That led to the crackdown in Kyiv’s streets in 2014.

Security forces opened fire on protesters in central Kyiv in February that year, killing dozens. Protesters held their ground, attracting public support in Europe and the United States. Mr. Yanukovych fled to Russia.

“In the hearts of Ukrainians and the eyes of the world, there is nothing strong about what Russia is doing,” Mr. Kerry said during his visit to Kyiv.

Within days, Mr. Putin ordered the invasion of Crimea, and he soon formally recognized it as a “sovereign and independent state.”

A slow-burn war in eastern Ukraine followed, with Kyiv battling a separatist movement supported by Russian weapons and troops. An estimated 13,000 people were killed over the next eight years.

Mr. Putin’s swift actions caught President Barack Obama off guard.

Mr. Obama vowed the United States would never recognize Russia’s annexation of Crimea and imposed economic sanctions, but his aides said in later accounts that he was skeptical of Ukraine’s corruption-ridden government.

And Mr. Obama said in a 2016 interview that a showdown with Mr. Putin over Ukraine would have been futile.

His administration gave more than $1.3 billion in assistance to Ukraine between 2014 and 2016, but Mr. Obama said no when his national security team, including Mr. Biden and Mr. Kerry, recommended sending weapons to Kyiv.

Among Mr. Obama’s defenders was Mr. Blinken, then the deputy secretary of state and now America’s top diplomat.

By sending military aid to Ukraine, “you’re playing to Russia’s strength, because Russia is right next door,” Mr. Blinken, then the deputy secretary of state, said in early 2015.

Any aid, he added, “is likely to be matched and then doubled and tripled and quadrupled by Russia.”

Neither the Obama administration nor its key European allies believed Ukraine was ready to join NATO. But tensions in the alliance were growing as Europeans sought to maintain trade ties and energy deals with Russia.

The division was captured in a phone call in which a senior State Department official profanely criticized European leaders’ approach to helping Ukraine. A leaked recording of the call was posted on YouTube in February 2014 in what was widely believed to be an attempt by Russia to stir up discord between the United States and Europe.

Yet as much as anything else, Ukraine was a costly distraction to Mr. Obama’s broader agenda.

“It was hard to reconcile the time and energy required to lead the diplomacy on Ukraine with the demands on the United States elsewhere around the world, especially after ISIS took over much of Iraq and Syria in the summer of 2014,” Derek H. Chollet, a senior Pentagon official at the time, wrote in a book about Mr. Obama’s foreign policy.

Mr. Chollet is now a senior counselor to Mr. Blinken at the State Department.

‘Do Us a Favor’

Volodymyr Zelensky, a former comedian, won a landslide victory in Ukraine’s presidential elections in April 2019 after campaigning on an anti-corruption pledge.

Once in office, he turned to ending the war in the Donbas region of eastern Ukraine through negotiations with Mr. Putin.

The new Ukrainian president “knew he needed the backing of the United States and the American president,” said William B. Taylor Jr., who started his second tour as ambassador to Ukraine that June after his predecessor, Marie L. Yovanovitch, was pushed out on Mr. Trump’s orders.

Mr. Zelensky tried to arrange a meeting with Mr. Trump at the White House. But Mr. Trump had negative views of Ukraine even before he took office, influenced partly by his former campaign chairman, Paul Manafort, who had made more than $60 million consulting for a Ukrainian political party backed by Russia.

Mr. Trump’s opinions were reinforced in meetings with Mr. Putin, whom he publicly admired, and Viktor Orban, the autocratic prime minister of Hungary.

And close associates of Mr. Trump, in particular Rudolph W. Giuliani, then his personal lawyer, were urging the president to get Mr. Zelensky to open two investigations: one into Mr. Biden, Mr. Trump’s main political opponent, for actions in Ukraine related to his son Hunter Biden’s business dealings; the other based in part on a debunked conspiracy theory that Ukraine, not Russia, had interfered in the 2016 election, to help Hillary Clinton. Mr. Trump embraced the theory because it undermined the finding of the U.S. intelligence community that Russia had interfered to help him.

But U.S. policy had been on a notably different track. Earlier, in December 2017, under pressure from his national security aides and Congress, Mr. Trump agreed to do what Mr. Obama would not: approve the sale of Javelin anti-tank missiles to Ukraine.

But in mid-2019, the White House froze $391 million in military aid to Ukraine, including the Javelins, to build leverage for Mr. Trump’s demands, congressional investigators later found. The move hobbled Ukraine’s war effort against Russia-backed separatists.

“For it to be held up, they couldn’t understand that,” Mr. Taylor said.

That set the stage for a fateful July 25 call between Mr. Trump and Mr. Zelensky. “I would like you to do us a favor,” Mr. Trump said. He requested the two investigations.

Mr. Zelensky and his aides were confused. “The rest of the U.S. government was very supportive of Ukraine,” Mr. Taylor said. “But from the top, the president had a different message and set of conditions.”

Mr. Zelensky scheduled a CNN interview for September to announce one or both of the investigations that Mr. Trump had requested to satisfy the American president. But the interview never happened because journalists had begun reporting on the hold on military aid, and lawmakers sympathetic to Ukraine had persisted in asking the White House about the suspended aid. On Sept. 9, three House committees announced investigations into the pressure campaign after reviewing a whistle-blower complaint citing the July call.

The Trump administration released the aid on Sept. 11.

Secretary of State Mike Pompeo met with Mr. Zelensky in Kyiv on Jan. 31, 2020, the first cabinet official to do so since the announcement of an impeachment inquiry into Mr. Trump the previous September. The Senate trial was underway.

Just days earlier, Mr. Pompeo had blown up at an NPR reporter in an interview, asking her to identify Ukraine on an unmarked map and yelling, “Do you think Americans care” about Ukraine? — using an expletive before “Ukraine.”

Yet in Kyiv, Mr. Pompeo stood next to Mr. Zelensky in the presidential palace and said the U.S. commitment to support Ukraine “will not waver.”

But the damage had been done, and Mr. Zelensky was unconvinced that the United States was a trusted ally, Ms. Yovanovitch said in an interview last month.

“Trying to use our national security policy in order to further President Trump’s personal and political agenda was not just wrong, but it was really detrimental to the bilateral relationship,” she said. “It colored how Zelensky handled foreign policy.”

With all the disruption, former U.S. officials said, Mr. Putin no doubt saw weakness in Washington.

Biden vs. Putin

Consumed by the pandemic and the economy, Mr. Biden did not prioritize Ukraine at first. But Mr. Blinken visited Kyiv in May 2021 with a message of support.

During a steady rain, Mr. Blinken joined Dmytro Kuleba, the Ukrainian foreign minister, on a walk to the Wall of National Remembrance, where photos of soldiers who had been killed in combat with Russia in the Donbas were displayed outside St. Michael’s monastery.

But he also went to Kyiv with some tough love, determined to press Ukraine to make political and economic changes — a core issue for Mr. Biden when he oversaw relations with the country as vice president.

Just before the visit, Mr. Zelensky’s government had replaced the chief executive of the largest state-owned energy company, whom Western officials had praised for his transparency. The State Department had chastised the move as “just the latest example” of Ukrainian leaders violating practices of good governance. In Kyiv, Mr. Blinken told reporters that he was urging Ukraine to strengthen itself by “building institutions, advancing reforms, combating corruption.”

Such concerns paled in the face of Russia’s growing military threat, which Washington was watching “very, very closely,” Mr. Blinken said. Mr. Putin had begun amassing troops along Ukraine’s borders. By fall, the number approached 100,000.

This past January, Mr. Blinken rushed back to Kyiv for more consultations before a hastily arranged meeting in Geneva with the Russian foreign minister, Sergey V. Lavrov, in a last-ditch attempt to avert war.

But Russia would not be deterred, and high-level contacts between Washington and Moscow have been severely limited ever since.

By contrast, Mr. Blinken speaks frequently to Mr. Kuleba to convey American support that, at least in terms of aid, has been greater than at any time in the three decades since Ukraine declared independence.

“The world is with you,” Mr. Blinken told him on March 5, stepping into Ukraine just a few feet beyond Poland’s border.

“We’re in it with Ukraine — one way or another, short run, the medium run, the long run,” he said.

Mr. Kuleba referred to an “unprecedented, swift reaction” to Russia’s invasion and thanked Mr. Blinken for the support.

“But,” he said, “it has to be continued.”