On Tuesday, Russia said it planned “to partially pull back troops” from where it had built up some 130,000 at the Ukrainian border, a move that comes after the United States warned last week that an “invasion could begin at any time.”

The Kremlin had appeared to be making all the preparations for war: moving more military equipment, medical units, even blood, to the front lines. That aggression could easily still spiral into the largest military conflict on European soil in decades. The West said it hasn’t seen any evidence troops are pulling back, though it’s a least a possible signal that Russian President Vladimir Putin may be willing to deescalate.

But the issues driving this standoff remain unresolved.

The conflict is about the future of Ukraine. But Ukraine is also a larger stage for Russia to try to reassert its influence in Europe and the world, and for Putin to cement his legacy. These are no small things for Putin, and he may decide that the only way to achieve them is to launch another incursion into Ukraine — an act that, at its most aggressive, could lead to tens of thousands of civilian deaths, a European refugee crisis, and a response from Western allies that includes tough sanctions affecting the global economy.

The US and Russia have drawn firm red lines that help explain what’s at stake. Russia presented the US with a list of demands, some of which were nonstarters for the United States and its allies in the North Atlantic Treaty Organization (NATO). Putin demanded that NATO stop its eastward expansion and deny membership to Ukraine, and that NATO roll back troop deployment in countries that had joined after 1997, which would turn back the clock decades on Europe’s security and geopolitical alignment.

These ultimatums are “a Russian attempt not only to secure interest in Ukraine but essentially relitigate the security architecture in Europe,” said Michael Kofman, research director in the Russia studies program at CNA, a research and analysis organization in Arlington, Virginia.

As expected, the US and NATO rejected those demands. Both the US and Russia know Ukraine is not going to become a NATO member anytime soon.

Some preeminent American foreign policy thinkers argued at the end of the Cold War that NATO never should have moved close to Russia’s borders in the first place. But NATO’s open-door policy says sovereign countries can choose their own security alliances. Giving in to Putin’s demands would hand the Kremlin veto power over NATO’s decision-making, and through it, the continent’s security.

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Now the world is watching and waiting to see what Putin will do next. An invasion isn’t a foregone conclusion. Moscow continues to deny that it has any plans to invade. But war, if it happened, could be devastating to Ukraine, with unpredictable fallout for the rest of Europe and the West. Which is why, imminent or not, the world is on edge.

The roots of the current crisis grew from the breakup of the Soviet Union

When the Soviet Union broke up in the early ’90s, Ukraine, a former Soviet republic, had the third-largest atomic arsenal in the world. The United States and Russia worked with Ukraine to denuclearize the country, and in a series of diplomatic agreements, Kyiv gave its hundreds of nuclear warheads back to Russia in exchange for security assurances that protected it from a potential Russian attack.

Those assurances were put to the test in 2014, when Russia invaded Ukraine. Russia annexed the Crimean Peninsula and backed a rebellion led by pro-Russia separatists in the eastern Donbas region. (The conflict in eastern Ukraine has killed more than 14,000 people to date.)

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Russia’s assault grew out of mass protests in Ukraine that toppled the country’s pro-Russian President Viktor Yanukovych (partially over his abandonment of a trade agreement with the European Union). US diplomats visited the demonstrations, in symbolic gestures that further agitated Putin.

President Barack Obama, hesitant to escalate tensions with Russia any further, was slow to mobilize a diplomatic response in Europe and did not immediately provide Ukrainians with offensive weapons.

“A lot of us were really appalled that not more was done for the violation of that [post-Soviet] agreement,” said Ian Kelly, a career diplomat who served as ambassador to Georgia from 2015 to 2018. “It just basically showed that if you have nuclear weapons” — as Russia does — “you’re inoculated against strong measures by the international community.”

But the very premise of a post-Soviet Europe is also helping to fuel today’s conflict. Putin has been fixated on reclaiming some semblance of empire, lost with the fall of the Soviet Union. Ukraine is central to this vision. Putin has said Ukrainians and Russians “were one people — a single whole,” or at least would be if not for the meddling from outside forces (as in, the West) that has created a “wall” between the two.

Ukraine isn’t joining NATO in the near future, and President Joe Biden has said as much. The core of the NATO treaty is Article 5, a commitment that an attack on any NATO country is treated as an attack on the entire alliance — meaning any Russian military engagement of a hypothetical NATO-member Ukraine would theoretically bring Moscow into conflict with the US, the UK, France, and the 27 other NATO members.

But the country is the fourth-largest recipient of military funding from the US, and the intelligence cooperation between the two countries has deepened in response to threats from Russia.

“Putin and the Kremlin understand that Ukraine will not be a part of NATO,” Ruslan Bortnik, director of the Ukrainian Institute of Politics, said. “But Ukraine became an informal member of NATO without a formal decision.”

Which is why Putin finds Ukraine’s orientation toward the EU and NATO (despite Russian aggression having quite a lot to do with that) untenable to Russia’s national security.

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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 on democracy and rule of law.

All of this has put Ukraine in an impossible 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.

Why Russia is threatening Ukraine now

The Russia-Ukraine crisis is a continuation of the one that began in 2014. But recent political developments within Ukraine, the US, Europe, and Russia help explain why Putin may feel now is the time to act.

Among those developments are the 2019 election of Ukrainian President Volodymyr Zelensky, a comedian who played a president on TV and then became the actual president. In addition to the other thing you might remember Zelensky for, he promised during his campaign that he would “reboot” peace talks to end the conflict in eastern Ukraine, including dealing with Putin directly to resolve the conflict. Russia, too, likely thought it could get something out of this: It saw Zelensky, a political novice, as someone who might be more open to Russia’s point of view.

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What Russia wants is for Zelensky to implement the 2014 and ’15 Minsk agreements, deals that would bring the pro-Russian regions back into Ukraine but would amount to, as one expert said, a “Trojan horse” for Moscow to wield influence and control. No Ukrainian president could accept those terms, and so Zelensky, under continued Russian pressure, has turned to the West for help, talking openly about wanting to join NATO.

Public opinion in Ukraine has also strongly swayed to support for ascension into Western bodies like the EU and NATO. That may have left Russia feeling as though it has exhausted all of its political and diplomatic tools to bring Ukraine back into the fold. “Moscow security elites feel that they have to act now because if they don’t, military cooperation between NATO and Ukraine will become even more intense and even more sophisticated,” Sarah Pagung, of the German Council on Foreign Relations, said.

Putin tested the West on Ukraine again in the spring of 2021, gathering forces and equipment near parts of the border. The troop buildup got the attention of the new Biden administration, which led to an announced summit between the two leaders. Days later, Russia began drawing down some of the troops on the border.

Putin’s perspective on the US has also shifted, experts said. To Putin, the chaotic Afghanistan withdrawal (which Moscow would know something about) and the US’s domestic turmoil are signs of weakness.

Putin may also see the West divided on the US’s role in the world. Biden is still trying to put the transatlantic alliance back together after the distrust that built up during the Trump administration. Some of Biden’s diplomatic blunders have alienated European partners, specifically that aforementioned messy Afghanistan withdrawal and the nuclear submarine deal that Biden rolled out with the UK and Australia that caught France off guard.

Europe has its own internal fractures, too. The EU and the UK are still dealing with the fallout from Brexit. Everyone is grappling with the ongoing Covid-19 pandemic. Germany has a new chancellor, Olaf Scholz, after 16 years of Angela Merkel, and the new coalition government is still trying to establish its foreign policy. Germany, along with other European countries, imports Russian natural gas, and energy prices are spiking right now. France has elections in April, and French President Emmanuel Macron is trying to carve out a spot for himself in these negotiations.

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Those divisions — which Washington is trying very hard to keep contained — may embolden Putin. Some experts noted Putin has his own domestic pressures to deal with, including the coronavirus and a struggling economy, and he may think such an adventure will boost his standing at home, just like it did in 2014.

Diplomacy hasn’t produced any breakthroughs so far — but it could

A few months into office, the Biden administration spoke about a “stable, predictable” relationship with Russia. That now seems out of the realm of possibility.

The White House is holding out the hope of a diplomatic resolution, even as it’s preparing for sanctions against Russia, sending money and weapons to Ukraine, and boosting US military presence in Eastern Europe. (Meanwhile, Macron met with Putin for five hours on February 7.)

Late last year, the White House started intensifying its diplomatic efforts with Russia. In December, Russia handed Washington its list of “legally binding security guarantees,” including those nonstarters like a ban on Ukrainian NATO membership, and demanded answers in writing. In January, US and Russian officials tried to negotiate a breakthrough in Geneva, with no success. The US directly responded to Russia’s ultimatums at the end of January.

In that response, the US and NATO rejected any deal on NATO membership, but leaked documents suggest the potential for new arms control agreements and increased transparency in terms of where NATO weapons and troops are stationed in Eastern Europe.

Biden reportedly reiterated many of those diplomatic proposals in his call with Putin Saturday — offers Putin has previously said ignored the key issues.

One thing Biden’s team has internalized — perhaps in response to the failures of the US response in 2014 — is that it needed European allies to check Russia’s aggression in Ukraine. The Biden administration has put a huge emphasis on working with NATO, the European Union, and individual European partners to counter Putin. “Europeans are utterly dependent on us for their security. They know it, they engage with us about it all the time, we have an alliance in which we’re at the epicenter,” said Max Bergmann of the Center for American Progress.

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What happens if Russia invades?

In 2014, Putin deployed unconventional tactics against Ukraine that have come to be known as “hybrid” warfare, such as irregular militias, cyber hacks, and disinformation.

These tactics surprised the West, including those within the Obama administration. It also allowed Russia to deny its direct involvement. In 2014, in the Donbas region, military units of “little green men” — soldiers in uniform but without official insignia — moved in with equipment. Moscow has fueled unrest since, and has continued to destabilize and undermine Ukraine through cyberattacks on critical infrastructure and disinformation campaigns.

It is possible that Moscow will take aggressive steps in all sorts of ways that don’t involve moving Russian troops across the border. It could escalate its proxy war, and launch sweeping disinformation campaigns and hacking operations. (It will also probably do these things if it does move troops into Ukraine.)

But this route looks a lot like the one Russia has already taken, and it hasn’t gotten Moscow closer to its objectives. “How much more can you destabilize? It doesn’t seem to have had a massive damaging impact on Ukraine’s pursuit of democracy, or even its tilt toward the West,” said Margarita Konaev, associate director of analysis and research fellow at Georgetown’s Center for Security and Emerging Technology.

And that might prompt Moscow to see more force as the solution.

There are plenty of possible scenarios for a Russian invasion, including sending more troops into the breakaway regions in eastern Ukraine, seizing strategic regions and blockading Ukraine’s access to waterways, and even a full-on war, with Moscow marching on Kyiv in an attempt to retake the entire country. Any of it could be devastating, though the more expansive the operation, the more catastrophic.

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A full-on invasion to seize all of Ukraine would be something Europe hasn’t seen in decades. It could involve urban warfare, including on the streets of Kyiv, and airstrikes on urban centers. It would cause astounding humanitarian consequences, including a refugee crisis. The US has estimated the civilian death toll could exceed 50,000, with somewhere between 1 million and 5 million refugees. Konaev noted that all urban warfare is harsh, but Russia’s fighting — witnessed in places like Syria — has been “particularly devastating, with very little regard for civilian protection.”

The colossal scale of such an offensive also makes it the least likely, experts say, and it would carry tremendous costs for Russia. “I think Putin himself knows that the stakes are really high,” Natia Seskuria, a fellow at the UK think tank Royal United Services Institute, said. “That’s why I think a full-scale invasion is a riskier option for Moscow in terms of potential political and economic causes — but also due to the number of casualties. Because if we compare Ukraine in 2014 to the Ukrainian army and its capabilities right now, they are much more capable.” (Western training and arms sales have something to do with those increased capabilities, to be sure.)

Such an invasion would force Russia to move into areas that are bitterly hostile toward it. That increases the likelihood of a prolonged resistance (possibly even one backed by the US) — and an invasion could turn into an occupation. “The sad reality is that Russia could take as much of Ukraine as it wants, but it can’t hold it,” said Melinda Haring, deputy director of the Atlantic Council’s Eurasia Center.

What happens now?

Ukraine has derailed the grand plans of the Biden administration — China, climate change, the pandemic — and become a top-level priority for the US, at least for the near term.

“One thing we’ve seen in common between the Obama administration and the Biden administration: They don’t view Russia as a geopolitical event-shaper, but we see Russia again and again shaping geopolitical events,” said Rachel Rizzo, a researcher at the Atlantic Council’s Europe Center.

The United States has deployed 3,000 troops to Europe in a show of solidarity for NATO and will reportedly send another 3,000 to Poland, though the Biden administration has been firm that US soldiers will not fight in Ukraine if war breaks out. The United States, along with other allies including the United Kingdom, have been warning citizens to leave Ukraine immediately. The US shuttered its embassy in Kyiv this week, temporarily moving operations to western Ukraine.

The Biden administration, along with its European allies, is trying to come up with an aggressive plan to punish Russia, should it invade again. The so-called nuclear options — such as an oil and gas embargo, or cutting Russia off from SWIFT, the electronic messaging service that makes global financial transactions possible — seem unlikely, in part because of the ways it could hurt the global economy. Russia isn’t an Iran or North Korea; it is a major economy that does a lot of trade, especially in raw materials and gas and oil.

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“Types of sanctions that hurt your target also hurt the sender. Ultimately, it comes down to the price the populations in the United States and Europe are prepared to pay,” said Richard Connolly, a lecturer in political economy at the Centre for Russian and East European Studies at the University of Birmingham.

Right now, the toughest sanctions the Biden administration is reportedly considering are some level of financial sanctions on Russia’s biggest banks — a step the Obama administration didn’t take in 2014 — and an export ban on advanced technologies. Penalties on Russian oligarchs and others close to the regime are likely also on the table, as are some other forms of targeted sanctions. Nord Stream 2, the completed but not yet open gas pipeline between Germany and Russia, may also be killed if Russia escalates tensions.

Putin himself has to decide what he wants. “He has two options,” said Olga Lautman, senior fellow at the Center for European Policy Analysis. One is “to say, ‘Never mind, just kidding,’ which will show his weakness and shows that he was intimidated by US and Europe standing together — and that creates weakness for him at home and with countries he’s attempting to influence.”

“Or he goes full forward with an attack,” she said. “At this point, we don’t know where it’s going, but the prospects are very grim.”

This is the corner Putin has put himself in, which makes a walk-back from Russia seem difficult to fathom. That doesn’t mean it can’t happen, and it doesn’t eliminate the possibility of some sort of diplomatic solution that gives Putin enough cover to declare victory without the West meeting all of his demands. It also doesn’t eliminate the possibility that Russia and the US will be stuck in this standoff for months longer, with Ukraine caught in the middle and under sustained threat from Russia.

But it also means the prospect of war remains. In Ukraine, though, that is everyday life.

“For many Ukrainians, we’re accustomed to war,” said Oleksiy Sorokin, the political editor and chief operating officer of the English-language Kyiv Independent publication.

“Having Russia on our tail,” he added, “having this constant threat of Russia going further — I think many Ukrainians are used to it.”