For Europe and NATO, a Russian Invasion Is No Longer Unthinkable

President Vladimir V. Putin of Russia once proclaimed the dissolution of the Soviet empire “the greatest geopolitical catastrophe of the 20th century.” At the time, back in 2005, few expected him to do anything about it.

But then came Russia’s occupation of Abkhazia and South Ossetia from Georgia in 2008, its backing for Ukrainian separatists and the annexation of Crimea in 2014 and, most resoundingly, the full-scale invasion of Ukraine in 2022.

Now, with the rise of former President Donald J. Trump, who in the past has vowed to leave NATO and recently threatened never to come to the aid of his alliance allies, concerns are rising among European nations that Mr. Putin could invade a NATO nation over the coming decade and that they might have to face his forces without U.S. support.

That could happen in as few as five years after a conclusion of the war in Ukraine, according to some officials and experts who believe that would be enough time for Moscow to rebuild and rearm its military.

“We have always kind of suspected that this is the only existential threat that we have,” Maj. Gen. Veiko-Vello Palm, the commander of the Estonian Army’s main land combat division, said of a possible Russian invasion.

“The past few years have also made it very, very clear that NATO as a military alliance, a lot of countries, are not ready to conduct large-scale operations — meaning, in simple human language, a lot of NATO militaries are not ready to fight Russia,” General Palm said during an interview in December. “So it’s not very comforting.”