For months, arms control advocates have argued for a series of steps to advance the pledge he made to pursue “a world without nuclear weapons.” An unequivocal no-first-use pledge would have been the boldest of those measures. They contend that as a practical matter no American president would use a nuclear weapon when so many other options are available.

Former Defense Secretary William J. Perry said in a recent interview, “It’s the right time,” noting that the pledge would formalize what has been America’s unspoken policy for decades.

But in the end, Mr. Obama seems to have sided with his current advisers, who warned in meetings culminating this summer that a no-first-use declaration would rattle allies like Japan and South Korea. Those nations are concerned about discussion of an American pullback from Asia prompted by comments made by the Republican presidential nominee, Donald J. Trump.

Defense Secretary Ashton B. Carter and Secretary of State John Kerry also expressed concern that new moves by Russia and China, from the Baltic to the South China Sea, made it the wrong time to issue the declaration, according to senior aides in the Defense and State Departments.