Can the West Stop Russia From Invading Ukraine? Here’s What You Need to Know.

It feels like a scene from the Cold War.

An unpredictable Russian president amassing thousands of troops on the border of a neighboring country, Ukraine. The threat of invasion. A possible bloody conflagration between East and West.

But what may seem like a perilous episode from a bygone era will be front and center this week in global diplomacy, when the United States, its NATO allies and Russia meet for various talks in Geneva, Vienna and Brussels aimed at averting another Russian incursion into Ukraine.

That potential military flare-up threatens to destabilize the already volatile post-Soviet region, buffeted by last week’s popular revolt in Kazakhstan. It would also have serious consequences for the security structure that has governed Europe since the collapse of the Soviet Union three decades ago.

In a move that has aggravated already tense relations between Washington and the Kremlin, Russia has mobilized more than 100,000 troops near its border with Ukraine. The United States has disclosed intelligence showing that Russia has a war plan envisioning an invasion force of 175,000 troops that Ukraine’s military, despite U.S.-provided equipment and training, would have little ability to stop.

On Friday, the NATO secretary general, Jens Stoltenberg, warned that “the risk of conflict is real.”

Russia has made a list of far-reaching demands: that NATO pledge to halt further eastward expansion and pull back its troops from NATO members bordering Russia; that Ukraine cease deployments of NATO weaponry; and that Ukraine yield to Russian terms for a settlement in the war in the eastern part of the country.

Essentially, Mr. Putin is seeking to redraw the post-Cold War boundaries of Europe, establishing a broad, Russian-dominated security zone and pulling Ukraine back into Moscow’s orbit by force, if necessary.

In the event of an invasion, the United States and its allies have threatened to impose a series of sanctions that would go far beyond those imposed in 2014, after the Russian annexation of Crimea. Mr. Putin warned that imposing new sanctions could lead to a “complete rupture” in relations with Washington.

Tensions between Ukraine and Russia have been simmering since 2014. That’s when Ukraine ousted its pro-Russian president and the Russian military crossed into Ukrainian territory, annexing Crimea and fomenting a rebellion by separatists in eastern Ukraine. A tenuous cease-fire was reached in 2015, but peace has been elusive amid a grinding war that has killed more than 13,000 soldiers and civilians.

The Kremlin’s position toward its neighbor has been hardening, as President Vladimir V. Putin of Russia has grown more insistent that Ukraine is fundamentally a part of Russia, culturally and historically. Concerns were raised in late October, when Ukraine used an armed drone to attack a howitzer operated by Russian-backed separatists in eastern Ukraine. Russia called the strike a destabilizing act that violated the cease-fire agreement.

What does Mr. Putin want?

Now 69 years old and edging toward the twilight of his political career, Mr. Putin is determined to burnish his legacy and to correct what he has long viewed as the greatest catastrophe of the 20th century: the disintegration of the former Soviet Union.

Asserting Moscow’s power over Ukraine, a country of 44 million people that was previously part of the Soviet bloc and shares a 1,200-mile border with Russia, is part of his aim of restoring what he views as Russia’s rightful place among the world’s great powers, along with the United States and China.

Mr. Putin has increasingly portrayed NATO’s eastward expansion as an existential threat to his country, and insists that Moscow’s military buildup is a reaction to Ukraine’s intensifying ties with the alliance. He appears intent on winding back the clock 30 years, to just before the collapse of the Soviet Union.

The timing of Russia’s troop mobilization is perhaps no coincidence. Mr. Putin is seeking to energize nationalist support at home amid a raging pandemic and a stumbling economy. Last year, opposition groups held some of the largest anti-Putin protests in years.

But while some analysts have portrayed Mr. Putin as a wily chess player adroitly manipulating the West, his latest gambit could backfire. NATO could reinforce its military presence in member countries bordering Russia, like the Baltics. And an invasion would invite punishing sanctions that could diminish his support in a country weary of foreign adventures that drain resources many would prefer to see spent at home.

In Ukraine, meanwhile, Moscow’s aggressive posture has further inflamed nationalist passions, with citizen militias preparing for a drawn-out guerrilla campaign in the event of a Russian occupation. And if Mr. Putin’s aim is to reassert Russia’s sphere of influence, invading Ukraine would further destabilize the post-Soviet region, where Russian troops are helping restore order in Kazakhstan and Belarus is still smoldering after an uprising in 2020.

How does the United States plan to respond?

In early December, President Biden made clear that his administration was not considering sending troops to Ukraine, since, among other reasons, Ukraine is not a member of the NATO alliance and does not come under its commitment to collective defense.

Instead, Mr. Biden said he would reinforce the American military presence in NATO countries that border Russia. And, referring to Mr. Putin, he promised that there would be “economic consequences like none he’s ever seen.” U.S. officials have hinted that Washington could be turning to its China playbook — potentially instituting sanctions that could deprive Russians of their beloved next-generation phones, laptops and other gadgets, and the military from advanced equipment. There is also the option of cutting Russia off from the international banking system, though analysts have said that is unlikely.

An intensifying conflict in Ukraine would test the resolve of the Biden administration as the United States has been working to restore confidence in America’s global leadership following the recent messy withdrawal from Afghanistan and its retrenchment from foreign engagements under President Donald J. Trump.

How the United States handles Russia and Ukraine will affect its ongoing efforts at rebuilding frayed ties with NATO allies after the Trump presidency, during which Mr. Trump declared the alliance “obsolete,” called member countries deadbeats and at first refused to explicitly endorse NATO’s bedrock mutual defense principle.

An escalating crisis in Ukraine also threatens to upend recent efforts by the United States and NATO to shift the alliance’s attention to the security challenge posed by China, and would push it back toward its traditional role of protecting Europe and, by extension, North America.

NATO’s response in the face of Russia’s attempt to neutralize the alliance will also help shape its geopolitical heft for years to come.

What’s at stake for Europe?

At stake for Europe is whether it can allow Mr. Putin to upend the security structure that has helped keep the peace on the continent since World War II. The conflict has also laid bare the weakness of the European Union and its failure as a foreign policy force in international relations.

The European Union doesn’t have a seat at the table at most of these talks, which, after all, are about European security, and it is seeking to be more involved.

With the departure of Chancellor Angela Merkel, who grew up in the east, speaks fluent Russian, and had developed a good working relationship with Mr. Putin, Europe lost an invaluable interlocutor with Moscow. Her successor, Chancellor Olaf Scholz, is less of a known quantity in foreign affairs, and is the head of a complicated coalition more critical about Russia than his own Social Democratic Party.

Europe has important trade ties with Russia, and would stand to lose far more than the U.S. from sanctions imposed after a Russian invasion of Ukraine. It is also dependent on Russian gas supplies, a weakness that Mr. Putin has exploited in past disputes.

Steven Erlanger in Brussels contributed reporting.