Supporters of arms control, as well as some of President Obama’s closest advisers, say their hopes for the president’s vision have turned to baffled disappointment as the modernization of nuclear capabilities has become an end unto itself.

“A lot of it is hard to explain,” said Sam Nunn, the former senator whose writings on nuclear disarmament deeply influenced Mr. Obama. “The president’s vision was a significant change in direction. But the process has preserved the status quo.”

With Russia on the warpath, China pressing its own territorial claims and Pakistan expanding its arsenal, the overall chances for Mr. Obama’s legacy of disarmament look increasingly dim, analysts say. Congress has expressed less interest in atomic reductions than looking tough in Washington’s escalating confrontation with Moscow.

“The most fundamental game changer is Putin’s invasion of Ukraine,” said Gary Samore, Mr. Obama’s top nuclear adviser in his first term and now a scholar at Harvard. “That has made any measure to reduce the stockpile unilaterally politically impossible.”