Though the Cold War was over, he argued that Russia still faced threats from nations on its periphery, including China and potentially Pakistan. But the Bush administration was reluctant to terminate a treaty that NATO nations regarded as a cornerstone of arms control and whose abrogation would have enabled the Russians to increase missile forces directed at the United States’ allies in Asia.

Since Mr. Obama has been in office, the Russians have insisted they want to keep the agreement. But in the view of American analysts, Russia has also mounted a determined effort to strengthen its nuclear abilities to compensate for the weakness of its conventional, nonnuclear forces.

At the same time, in his State of the Union address last year, Mr. Obama vowed to “seek further reductions in our nuclear arsenals,” a goal American officials at one point hoped might form part of Mr. Obama’s legacy.

But administration officials and experts outside government say Congress is highly unlikely to approve an agreement mandating more cuts unless the question of Russian compliance with the medium-range treaty is resolved.

“If the Russian government has made a considered decision to field a prohibited system,” Franklin C. Miller, a former defense official at the White House and the Pentagon, said, “then it is the strongest indication to date that they are not interested in pursuing any arms control, at least through the remainder of President Obama’s term.”

It took years for American intelligence to gather information on Russia’s new missile system, but by the end of 2011, officials say it was clear that there was a compliance concern.

There have been repeated rumors over the last year that Russia may have violated some of the provisions of the 1987 treaty. But the nature of that violation has not previously been disclosed, and some news reports have focused on the wrong system: a new two-stage missile called the RS-26. The Russians have flight-tested it at medium range, according to intelligence assessments, and the prevailing view among Western officials is that it is intended to help fill the gap in Russia’s medium-range missile capabilities that resulted from the 1987 treaty. The treaty defines medium-range missiles as ground-launched ballistic or cruise missiles capable of flying 300 to 3,400 miles.