Most European leaders — especially the Germans — believe other weapons systems deter the Russians, including air- and ground-launched missiles. For them, Mr. Trump’s decision to abandon one of the few remaining treaties controlling nuclear weapons fits a narrative of “America First” at the expense of existing, long-term alliances, like NATO — and is the latest in a series of abandoned agreements, from the Paris accord on climate to the Iranian nuclear deal.

In this case, they see few advantages from leaving the treaty. Carl Bildt, a former Swedish prime minister, called the move “a gift to Russia that exposes Europe to a growing nuclear threat,” because as the United States enters an arms race, “Russia can quickly deploy new weapons in numbers.”

The German foreign minister, Heiko Maas, called the decision regrettable, noting that it “poses difficult questions for us and for Europe” since it is the Europeans who are in range of the Russian missiles, not the United States.

Mr. Gorbachev, unsurprisingly, decried the Trump decision as reckless, asking: “Do they really not understand in Washington what this can lead to?”

Moreover, the Europeans believe Mr. Trump’s strategy — praising Mr. Putin when the two appear together as they did in Helsinki, then letting his aides step up pressure — is, if anything, emboldening the Russian leader. They were stunned to see Russia send a hit squad to Britain to try to kill a former Russian intelligence officer, Sergei V. Skripal, despite having exchanged him in a spy-swap years before. And Russia continues to freely meddle in European politics, most recently trying to block the accession of Macedonia to both NATO and the European Union.

But the European reaction has been disorganized. While NATO countries have put more troops in Baltic nations and Poland, and are preparing a huge military exercise in the North Atlantic, there is no agreed-on strategy over what red lines should be set to respond to Russian activity. Nowhere is that clearer than in the realm of cyberwarfare, where Europeans are spending more money on collective defense, but NATO has no offensive capability and no agreement about what kind of interference by the Russians calls for a response.

For his part, Mr. Putin has calibrated his actions with care. He denies that the Russian deployment of what the West calls an SSC-8 missile violates the treaty. And he has accused the United States — long before Mr. Trump was elected — of violating the treaty itself, arguing that antimissile batteries it has placed in Europe could be used to fire other missiles that violate the ban on weapons that can reach 300 to 3,500 miles.