When tens of thousands of Russian troops started moving toward the Ukrainian border late last year, Russian President Vladimir Putin effectively issued an ultimatum: They won’t go home until he had “concrete agreements prohibiting any further eastward expansion of NATO.”

This week, as the US and Russia exchange formal diplomatic letters, Secretary of State Antony Blinken emphasized that “NATO’s door is open, remains open, and that is our commitment.”

But few have been asking why the North Atlantic Treaty Organization (NATO) would want to move east in the first place. What was once a Cold War security pact has become a 21st-century organization with global military commitments and ever more member countries from Eastern Europe. Members of the alliance didn’t always foresee its expansion and, three decades ago, some of America’s most renowned foreign policy thinkers argued that NATO should be nowhere near Ukraine.

Ukraine is a former Soviet republic. It isn’t joining NATO anytime soon, and President Joe Biden has said as much. Still, NATO’s open-door policy — the alliance’s foundational principle that any qualified European country could join — cuts both ways. To the West, it’s a statement of autonomy; to Russia, it’s a threat. The core of the NATO treaty is Article 5, a commitment that an attack on any country is treated as an attack on the entire alliance — meaning any Russian military engagement with a hypothetical NATO-member Ukraine would theoretically bring Moscow into conflict with the US, the UK, France, and the 27 other NATO members.

The prospect of Ukraine and Georgia joining NATO has antagonized Putin at least since President George W. Bush expressed support for the idea in 2008. “That was a real mistake,” said Steven Pifer, who from 1998 to 2000 was ambassador to Ukraine under President Bill Clinton. “It drove the Russians nuts. It created expectations in Ukraine and Georgia, which then were never met. And so that just made that whole issue of enlargement a complicated one.”

No country can join the alliance without the unanimous buy-in of all 30 member countries, and many have opposed Ukraine’s membership, in part because it doesn’t meet the conditions to join. All of this has put Ukraine in an untenable position: an applicant for an alliance that wasn’t going to accept it, while irritating a potential opponent next door, without having any degree of NATO protection.

Revisiting NATO’s own history is not to justify Putin’s revanchism and threats to democracy. It is certainly true that he is a repressive leader who has annexed neighbors and funded separatists, cracked down on activists and allegedly poisoned enemies. Some experts say that his criticism of NATO expansion is a mere pretext. Still, the stakes of NATO’s presence on Russia’s borders and potential expansion are high, and at least in today’s Washington, few question that presence.

“The open-door policy is the one that maximizes friction with Russia, which has culminated in the crisis we have now,” said Mary Sarotte, a historian of international relations at Johns Hopkins University. “I don’t think Vladimir Putin is primarily interested in historical accuracy, but I believe he is genuinely aggrieved at the way the post–Cold War order includes no stake for Russia.”

So how did it become an article of faith in Washington that NATO would expand in its membership and its purpose?

Debating NATO’s future in the ’90s

As the Soviet Union cracked up, it wasn’t certain that NATO would stick around either.

“During the Cold War, NATO had a mission that was clear and tight, and could be put on a bumper sticker,” said Rajan Menon of the research group Defense Priorities. The alliance existed, he explained, “to deter and defeat the Warsaw pact,” the countries aligned with collapsing Soviet Russia. Its mission was in doubt after the Cold War — so much so that the president of the dissolving Soviet Union, Mikhail Gorbachev, even asked about his country joining NATO.

Even the shape of the US role in Europe was uncertain. After the fall of the Berlin Wall, the American public was more concerned about domestic policy. Bill Clinton had been elected with a campaign slogan of “It’s the Economy, Stupid,” and without experience as a statesman, he seemed likely to restrain America’s global ambitions.

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As Clinton became a frequent traveler to Russia and quickly plunged into statecraft, the promotion of democracy in Europe emerged as a primary US foreign policy goal. But it wasn’t clear that a military alliance like NATO would be the best way to advance that.

A debate over NATO’s merits erupted in Washington in the ’90s. George Kennan, the eminent architect of the Soviet “containment” strategy and a former ambassador to the Soviet Union, wrote in 1997 that expanding NATO would be a “fateful error” because it would “inflame the nationalistic, anti-Western and militaristic tendencies in Russian opinion.” Kennan was far from alone in his criticism, as journalist Peter Beinart noted this week:

Thomas Friedman, America’s most prominent foreign policy columnist, declared it the “most ill-conceived project of the post-Cold War era.” Daniel Patrick Moynihan, widely considered the most erudite member of the US Senate, warned, “We have no idea what we’re getting into.”

Meanwhile, military leaders saw enlargement as detrimental to US interests, the Congressional Budget Office saw it as too expensive, and, later, intelligence agencies outright opposed adding Ukraine and Georgia. Clinton’s Secretary of Defense William Perry wrote in his memoir that he nearly resigned over enlargement.

The nascent European Union might have been the channel to consolidate democratic development in post-Soviet countries. Or Europe could have been engaged through the multinational Organization for Economic Cooperation and Development, or even through a focus on closer relationships with individual countries.

But Washington chose NATO.

In the early ’90s, that generation of national security operatives weren’t prepared to forfeit leverage in Europe. “NATO had to find something to do or go out of business, and these people who grew up all their lives alongside it would not let it go out of business,” said Barry Posen, a political scientist at MIT.

Jenonne Walker, who served in the Clinton White House, said she was among the minority who would have preferred the European Union as the mechanism for US engagement. “Almost everyone in the establishment wanted it to be through NATO, because that was where our influence was deemed to be greatest,” she said.

Clinton first floated a program that would be a gateway to NATO membership, called the Partnership for Peace, but that was ultimately dropped. By 1994, NATO said it “would welcome NATO enlargement that would reach to democratic states to our East,” and Poland, the Czech Republic, and Hungry would be the first to join.

Electoral politics of the moment also reinforced Clinton’s decision to back NATO expansion. Republicans had won on that platform in the ’94 midterms. Ahead of the 1996 presidential election, “the domestic side of the White House” believed that growing the alliance would “play good with Polish American, Baltic American, Hungarian American communities,” explained Pifer.

President Clinton, his national security adviser Tony Lake, and Secretary of State Warren Christopher were optimistic that NATO could branch out, with the possibility of a constructive relationship between NATO and Russia. As that trifecta supported adding member states, the NATO alliance became an organizing principle of US foreign policy going forward.

NATO enlargement reoriented America in the world

Initially, it took much political maneuvering for the newly united Germany to join NATO. The alliance added more eastern bloc counties in the late ’90s and early 2000s. The Baltic countries of Estonia, Latvia, and Lithuania on the northern end of Russia’s western border joined NATO in 2005 without much fuss from Russia.

As it grew, NATO became a vehicle to address new global issues that worried US leaders. “Enlarging NATO becomes the gift that keeps on giving,” said Joshua Shifrinson, an international relations scholar at Boston University. “It was a way of incentivizing liberalization in countries that had been in the Communist bloc, showing that the US still has a mission in Europe, and a way of the US projecting power and checking alternative systems like the European Union.”

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During the Cold War, NATO never engaged in military operations. But amid the Yugoslav conflict and Kosovo war of the ’90s, the alliance enforced a no-fly zone, then deployed a peacekeeping force, and in 1999 dropped hundreds of bombs on Yugoslavia. The whole process was delayed and disorganized, according to diplomats, and exposed NATO’s inadequacies in dealing with a hot war.

That pushed Clinton to embrace NATO further. “Our inaction was making NATO look weak and irrelevant,” said Walker, who went on to serve as ambassador to the Czech Republic from 1995 to 1998. “And the line in the halls of power in Washington was, ‘We have to enlarge NATO to save it, to make it look as though it were dynamic and on the move and not stagnant.’”

By taking on new military roles, the institution created new imperatives for itself. In the 2000s, NATO went to the front lines: fighting in Afghanistan after the September 11 attacks and training Afghan forces starting in 2003, countering piracy in waters near Somalia, and then in a military intervention that was meant to protect civilians in Libya and went much further than its United Nations–approved mandate in toppling the tyrant Muammar Qaddafi.

Now, America’s foreign policy establishment is dominated by people who are even more committed to the alliance’s power than those who saved it in the 1990s. NATO’s existence and enlargement is a baseline assumption. Bosnia and Herzegovina, Georgia, and Ukraine aspire to join.

That has consequences. James Dobbins, who served as a senior diplomat in Europe during the ’90s and 2000s, says that a commitment to NATO expansion has limited Biden’s options. “It’s particularly out of tune — the idea that the United States should expand its defense perimeter to a half-dozen countries in Europe, when we should be shifting our focus on China,” Dobbins said.

At its core, this is about US power and how it has changed since the Soviet Union’s end. “It’s become a conversation about whether the US should be out in the world defending human rights and spreading democracy,” said Emma Ashford of the Atlantic Council. “Is the US out in the world to protect its own security or to be a crusading force for good?”

The Biden administration will now have to find its own answer.

Update, January, 27, 2021, 8:30 am: This story has been updated to include comments this week from Secretary of State Antony Blinken.