WASHINGTON — The tense talks this week among the United States, Russia and European members of the North Atlantic Treaty Organization have made one thing clear: While the Biden administration insists it will not allow Moscow to quash Ukraine’s ambitions to join NATO, it has no immediate plans to help bring the former Soviet republic into the alliance.

If Ukraine were a NATO member, the alliance would be obligated to defend it against Russia and other adversaries. U.S. officials say they will not appease President Vladimir V. Putin of Russia by undermining a policy enshrined in NATO’s original 1949 treaty, which grants any European nation the right to ask to join.

“Together, the United States and our NATO allies made clear we will not slam the door shut on NATO’s open door policy — a policy that has always been central to the NATO alliance,” Wendy R. Sherman, the deputy secretary of state, said on Wednesday.

But France and Germany have in the past opposed Ukraine’s inclusion, and other European members are wary — a deal breaker for an alliance that grants membership only by unanimous consent. American and Russian leaders know this. With Russian troops amassed on Ukraine’s eastern border, some current and former American and European officials say Mr. Putin might just be raising the NATO issue as a pretext for an invasion.

Michael McFaul, a former U.S. ambassador to Russia, has suggested that Mr. Putin is trying to distract from more urgent matters. “Everybody’s talking about NATO expansion,” Mr. McFaul said on a podcast by the Center for a New American Security that was released on Tuesday. “Suddenly, we’re debating this issue that wasn’t even an issue. That’s a great advantage to him.”

Like European leaders, President Biden remains uninterested in Ukrainian membership in NATO. Here are four reasons.

Biden has grown skeptical of expanding U.S. military commitments.

After the collapse of the Soviet Union, Mr. Biden successfully urged NATO to accept Poland, Hungary and the Czech Republic as member states in the late 1990s. The top Democrat on the Senate Foreign Relations Committee at the time, Mr. Biden said that turning the former Cold War adversaries into allies would mark the “beginning of another 50 years of peace” for Europe. He added that the move would right a “historical injustice” perpetrated by Stalin.

But over the course of two decades of war in Iraq and Afghanistan, experts said, Mr. Biden’s fervor for expanding NATO cooled considerably. In 2004, seven Eastern European countries joined the alliance, and in 2008, President George W. Bush pushed NATO to issue a declaration that Ukraine and Georgia would become members despite reservations from U.S. intelligence agencies. However, the alliance has never offered either country a formal action plan to join, a necessary step for them to do so.

As recently as June, Secretary of State Antony J. Blinken told senators that “we support Ukraine membership in NATO.” Mr. Biden, however, has been far more circumspect in his public comments and “has soft-pedaled talk of extending NATO membership to Ukraine,” two foreign policy scholars, Joshua Shifrinson and Stephen Wertheim, wrote in September in Foreign Affairs.

In 2014, as vice president, Mr. Biden told officials in Ukraine during a visit there that any U.S. military support would be small, if given at all, according to a biography of Mr. Biden by Evan Osnos, a New Yorker writer who was on the trip. Russia had just invaded and annexed the Ukrainian peninsula of Crimea, and Ukrainian officials were unhappy with Mr. Biden’s message.

“We no longer think in Cold War terms,” Mr. Biden told Mr. Osnos, adding that “there is nothing that Putin can do militarily to fundamentally alter American interests.”

Last June, Mr. Biden told journalists at NATO headquarters in Brussels that “school is out on that question” when asked whether Ukraine could join the alliance.

Biden wants Ukraine to improve its political and legal systems.

To meet one of the three main criteria for entry into NATO, a European nation must demonstrate a commitment to democracy, individual liberty and support for the rule of law. While Ukrainian leaders say they have met that threshold, some American and European officials argue otherwise.

In a 2020 analysis, Transparency International, an anticorruption watchdog, ranked Ukraine 117th out of 180 countries on its corruption index, lower than any NATO nation.

Officials in European nations with stronger liberal governance — notably in Sweden and Finland — have also floated the possibility of joining NATO, despite years of determined nonalignment. That is a discussion “we are ready to do,” Victoria J. Nuland, the State Department’s under secretary for political affairs, told journalists on Tuesday. “Obviously, they are longtime, established, stable democracies.”

She signaled that might not be the case with Ukraine. “That conversation would be slightly different than it is with countries that are making the transition to democratic systems and dealing with intensive problems of corruption and economic reform and democratic stability, etc.,” Ms. Nuland said.

Her comments echoed those of Mr. Biden on his 2014 visit to Ukraine. “To be very blunt about it, and this is a delicate thing to say to a group of leaders in their house of parliament, but you have to fight the cancer of corruption that is endemic in your system right now,” Mr. Biden told Ukrainian officials then.

Some Western officials also question whether Ukraine could meet a second set of criteria: contributing to the collective defense of NATO nations. But Ukraine sent troops to the U.S.-led wars in Iraq and Afghanistan.

“There are steps that Ukraine needs to take,” Jen Psaki, the White House press secretary, said in September after President Volodymyr Zelensky of Ukraine met with Mr. Biden in the Oval Office. “They’re very familiar with these: efforts to advance rule of law reforms, modernize its defense sector and expand economic growth.”

NATO wants to avoid greater Russian hostility.

After annexing Crimea, Mr. Putin invaded eastern Ukraine and gave military aid to a separatist insurgency there. He did something similar in Georgia in 2008. The message has been clear: If these two nations join NATO, the United States and European countries will have to grapple directly with ongoing Russian-fueled conflicts.

Russia could also impose other costs on Europe, such as withholding gas exports. And Germany and many other NATO nations prefer to choose their battles with Russia, given its proximity and Mr. Putin’s aggressive nature. They know he and other Russian officials are obsessed with Ukraine.

Given all that, Ukraine would almost certainly be unable to meet the third main criterion to join NATO: approval from all 30 members.

“The principal objection would be: Does such a move actually contribute to the stability in Europe, or would it contribute to destabilization?” said Douglas E. Lute, a former U.S. ambassador to NATO. “I think it’s indisputable there wouldn’t be consensus among the 30 members, even though all allies agree that Ukraine has the right to aspire to become a NATO member.”

Stephen M. Walt, a professor of international affairs at the Harvard Kennedy School of Government, said that even in the 1990s, when NATO enlargement was first proposed, many prominent American strategists opposed it for this reason. “That was the concern all along — it wouldn’t be easy to do this in a way that wouldn’t threaten Russia,” he said.

Ukrainian leaders have waffled on NATO membership.

Ukrainian leaders have not always pushed hard to join NATO, and that has shaped the American approach.

Former President Viktor Yushchenko wanted entry into the alliance, but Ukrainians became more reluctant after Russia invaded Georgia. His successor, Viktor Yanukovych, dropped any drive for membership and promoted closer ties with Russia, even agreeing to allow Moscow to continue leasing a Black Sea naval port in Crimea.

During the Obama administration, American officials encouraged Ukraine to sign a formal association agreement with the European Union rather than to try to join NATO. Mr. Putin pressured Mr. Yanukovych to reject the agreement, which led to the Euromaidan protests in 2013 that eventually ousted Mr. Yanukovych.

“A lot of the U.S. policy has been quite reactive due to circumstances,” said Fiona Hill, a Russia expert at the Brookings Institution who was a senior director for Europe and Russia on the National Security Council under President Donald J. Trump. “It has also changed due to changes in Ukraine itself toward this.”

“By now, you’ve got much more sentiment in Ukraine for joining NATO,” she added.

Mr. Zelensky has pressed Mr. Biden repeatedly on membership, including during his visit to the White House in September. “I would like to discuss with President Biden here his vision, his government’s vision of Ukraine’s chances to join NATO and the time frame for this accession, if it is possible,” he said as he sat next to Mr. Biden.

Mr. Biden blew past those comments without responding.