For years, the threat of a Russia intent on expansion has loomed over much of Europe. It is not just the war in Ukraine. Moscow has also stationed military forces in friendly countries and fanned the flames of destabilizing political movements across the continent. Now, as the Trump administration strikes a conciliatory tone toward Russia and pulls away from some of America’s oldest allies, European leaders who long enjoyed the backing of the powerful U.S. military sense an extraordinary moment of vulnerability. For nearly 80 years, the United States has been the defensive and financial cornerstone of democratic security pacts created after World War II, in large part to keep the Soviet Union from marching across Europe. And even now, more than 80,000 American troops are deployed in Europe, some in a show of force right on Russia’s doorstep. But with a widening chasm in the trans-Atlantic relationship, the Russia that emerged from the collapse of the Soviet Union has been given a big opportunity. Russia’s president, Vladimir V. Putin, has made clear that his interests do not stop at Ukraine. Experts say he envisions a Soviet-style sphere of influence extending deep into Europe. It is reminiscent of the Cold War, when Germany was split between Russia and the West, and about half a dozen countries were pulled behind the Iron Curtain. The withdrawal of the American nuclear umbrella has protected Europe for generations, but both Britain and France have raised the possibility of extending their own arsenals to cover the continent. Both Finland and Sweden, which had tried for decades to steer clear of superpower gamesmanship, joined NATO after Russia invaded Ukraine in 2022. And Russian aggression has also ended up strengthening NATO. Mr. Putin has insisted that the military alliance withdraw from much of Central and Eastern Europe. Instead, it has grown. Moscow’s forays into Europe are not confined to the military. It has also sought to weaken its European adversaries by undermining their political systems. In some cases, Russia is believed to have directly financed the election campaigns of candidates friendly to its interests. Other times it has fomented support for far-right or Euroskeptic parties, including through disinformation campaigns. But pockets of support remain. Serbia and Serb-led areas in Bosnia and Herzegovina maintain longstanding religious and cultural ties to Russia even as they try to maintain a balance with the European Union. Yet nowhere in Europe does Russia have as much of a political foothold as in Belarus, in a relationship that the Council on Foreign Relations has termed “an axis of autocracy.” The American nuclear umbrella has protected Europe for generations, but both Britain and France have raised the possibility of extending their own arsenals. The shift in tone from Washington has injected a new sense of urgency in Europe. European leaders are increasing their defense spending and spearheading their own military action. And they’re doing it much more quickly than they did during the Cold War.!—-<\!----> Yet nowhere in Europe does Russia have as much of a political foothold as in Belarus, in a relationship that the Council on Foreign Relations has termed “an axis of autocracy.” The American umbrella has protected Europe for generations, but both Britain and France have raised the possibility of extending their own arsenals. And the idea that they need to pull together in the name of security against Russia is gaining currency across the continent as well. Across the continent, amid the upheaval of the refugee crisis and the response to terrorism, polls show that more Europeans are now favoring a shared security strategy with the US.