Where will Putin stop? As Ukraine holds on, Eastern Europe holds its breath

Where does Russian President Vladimir Putin plan to stop?

That is the question that has thrust the U.S. and its allies into an uncertain new era, cleaving a divide between East and West not seen since the Cold War and conjuring up fears of worst-case scenarios that were difficult to comprehend days ago.

Putin always knew that the U.S. and Western Europe would not step in militarily when he launched his invasion last week; the White House and other governments have always made it clear that they would not go toe to toe with Russia, the country with the world’s largest nuclear arsenal, over the defense of Ukraine, which is not a member of NATO.

But the same is not true for Poland, Lithuania, Latvia and Estonia, which are all members of the alliance. That means, in theory, that they would enjoy the protection of NATO’s Article 5 — the tenet that suggests that all allies, including the U.S., would come to the aid of any member that comes under attack.

For the Baltic states, in particular, Putin's threats against Ukraine were seen as a de facto threat. He said he sees Ukraine as historically Russian land — a territorial claim that he could make over them, too.

That trio of northern European countries — Lithuania, Latvia and Estonia — does not have the same cultural and linguistic ties to Russia that Ukraine does. But Moscow ruled them for much of the past 200 years, first under the Russian Empire and then under the Soviet Union.

They all voted for independence from the USSR in 1991 and joined NATO in 2004 — a development on Putin's long list of grievances against the West.

Putin's aim of "reversing history and going back to Russia’s status as it was over 100 years ago also directly affects other neighbors,” Keir Giles, a senior consulting fellow at Chatham House, a London think tank, said last week. “This means Estonia, Latvia, Lithuania, Poland and Finland.”

If his campaign succeeds in Ukraine, Putin could turn his attention to Moldova or Georgia, two former Soviet republics that today have breakaway regions occupied by Russian troops, said Karin von Hippel, a nonpolitical senior adviser at the State Department during the Obama administration.

Neither Moldova nor Georgia are NATO members. But if Putin "starts to slowly expand his empire, there will be several other places that are in NATO that are going to be getting extremely stressed out," said von Hippel, who is now the director-general of the Royal United Services Institute in London.

Before Russia invaded Ukraine, that scenario seemed unthinkable.

In November, British Prime Minister Boris Johnson told lawmakers that “we have to recognize that the old concepts of fighting big tank battles on European land mass are over.”

By launching his attack on a European democracy, Putin has shattered that preconception.

Russian forces have met stronger-than-expected resistance in Ukraine. Western intelligence officials and experts believe Putin’s plan is to take control of the capital, possibly to install a new regime more amenable to Moscow.

Putin has also further entrenched himself in Belarus, an ally that borders Poland and Lithuania, which looked more like a satellite state after it hosted some of the Russian troops that rolled toward Kyiv.

There is great uncertainty about how far west in Ukraine Putin intends to go — and Washington’s allies in the Baltics are alarmed that he may not stop at Ukraine at all.

Lithuania announced a state of emergency Thursday, and Estonia and Latvia called for urgent security talks. All three Baltic countries have said they will ban Russian commercial flights from their airspace.

Leaders of those governments have in recent weeks shuttled to European capitals to warn that failure to deter Putin in Ukraine could embolden him to extend his reach elsewhere.

"The battle for Ukraine is a battle for Europe," Lithuanian Foreign Minister Gabrielius Landsbergis warned this month. "If Putin is not stopped there, he will go further."

Others are not quite so concerned.

Adm. James Stavridis, a former supreme allied commander of NATO, pointed to the alliance's relative military strength and said last week that he was confident that "Putin will not cross a NATO border the way he’s gone across this Ukrainian border."

Still, for the first time in its 73-year history, NATO said in the wake of Putin's invasion that it was deploying its combat-ready “response force” to Eastern Europe and reminded Russia that NATO's commitment to Article 5 was “iron-clad.”

It has been a clarifying moment for the trans-Atlantic alliance, which struggled to find relevance after the Cold War and in recent years has been called "obsolete" by former President Donald Trump and "brain dead" by French President Emmanuel Macron.