It was only 11 months ago that the American energy secretary - Ernest J. Moniz, a former M.I.T. professor who has championed scientific programs that would bury the Cold War competitions between the United States and Russia - went to Vienna to sign the agreement, an indication of how recently the Obama administration believed it had a chance of building on a quarter-century of gradual integration of Russia with the West, The New York Times writes.
Handshakes and congratulations exchanged with Mr. Moniz's Russian counterpart, Sergey V. Kirienko, sealed an arrangement that would let Russian scientists into, among other places, the heart of the American nuclear complex at Los Alamos National Laboratory in New Mexico, where the first atomic bomb was constructed 70 years ago, and a dozen sister laboratories devoted to the making of the American nuclear arsenal. In return, American scientists would be allowed deep into Russian nuclear facilities, including the birthplace of the Soviet bomb.
The Energy Department's announcement of the deal also highlighted its potential for "defense from asteroids," shorthand for a proposal to recycle a city-busting warhead that could be aimed at an incoming earth-destroyer - a plot Hollywood had imagined 15 years before in two far-fetched thrillers, "Armageddon" and "Deep Impact," in which Bruce Willis and Ben Affleck, among others, saved humanity.
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Today, the real-life accord is on ice. This year, the Energy Department canceled nuclear meetings, symposia and lab visits with Russia.
Daniel B. Poneman, the deputy energy secretary, said that Russia's annexation of Crimea in March had prompted the decision to freeze the accord.
"We've made it very clear that this is not a time for business as usual," Mr. Poneman said Friday in his office. He added, however, that the Energy Department continued to work with Russia on the security of atomic materials.
American officials and experts say the decision will limit how much each side knows about the other's capabilities and intentions after more than two decades in which American and Russian nuclear scientists worked alongside one another. Those programs let once-bitter rivals, locked in the ultimate arms race, take each other's measure and deepen relationships, reducing the chances for deadly miscalculation and technological surprise.
Now, both sides are slipping back toward habits reminiscent of the Cold War. The joint atomic projects have declined substantially. Last week, Washington accused Moscow of violating a major arms treaty on missile technology. After the negotiation of the modest New Start treaty in 2010, progress toward another round of nuclear warhead reductions is dead in the water and unlikely to be revived during President Obama's term in office.
Satellite photographs released publicly by American intelligence agencies purport to show the movement of heavy arms across the Ukrainian border from Russia - evidence reminiscent of the kind released by the United States during conflicts half a century ago over Cuba and Berlin.
Perhaps most startling is not the direction of these steps, but their speed: As recently as January, the two sides were meeting regularly on joint arms control and scientific programs. The cancellations show how rapidly Mr. Obama has moved from a strategy that assumed Russia's continued interest in cooperation to one that assumes that President Vladimir V. Putin of Russia is out to take as much territory and control as he can, and that letting Russian scientists into America's nuclear complex is unwise.
For Mr. Obama, the motivation for negotiating the accord clearly had much less to do with asteroid destruction than geopolitics.
The United States' 20-year effort to secure Soviet nuclear materials was winding down. Siegfried S. Hecker, a former director of the Los Alamos weapons lab who has worked closely with Russian scientists, said the agreement had promised a new phase of teamwork and technical collaboration.
"It was an attempt to get back to good scientific cooperation," Mr. Hecker said. "Unfortunately, such things were struggling before Ukraine and have gone nowhere since."
That is not true of every effort at cooperation. Americans and Russians are still working alongside each other, though increasingly at cross-purposes, on the Iranian nuclear negotiations. The United States still needs its astronauts to ride to the International Space Station on Russian rockets, and it wants to keep buying Russian engines for its missiles.
Even so, at a moment when the White House is imposing sanctions and working to counter the flow of weapons into Ukraine, it might be difficult to justify an exchange of nuclear scientists. But some experts say it is when times are tense that such midlevel interchanges are the most critical.
"The idea of having thick relations with Russian nuclear scientists is a good idea," said Graham Allison, a Harvard professor who negotiated some early deals on securing the Soviet arsenal during the Clinton administration, the peak of cooperation between American and Russian nuclear weapons scientists. "People get to know each other, work on joint projects, and there is a basis for conversation and cooperation."
That was part of the impetus, after the collapse of the Soviet Union, behind the West's effort to fund new projects for Russian nuclear scientists. Keeping them busily employed, the theory went, made it less likely that they would sell their expertise to Iran, North Korea or a terrorist group with nuclear ambitions.
But the agreement last fall went far beyond that: It promised cooperation on complex, if peaceful, nuclear programs, including wide Russian access to the American nuclear complex.
While it would have allowed both sides to exclude "sensitive" military sites, it listed 137 American installations at 15 locations from coast to coast, including the centers for nuclear weapons design at Los Alamos and in Albuquerque and Livermore, Calif.
The 25 installations at Los Alamos included firing sites, the Warhead Verification Test Lab, the Sigma Complex for materials development and the Dual-Axis Radiographic Hydrodynamic Test Facility, a giant X-ray machine that can peer into bomb processes.
The accord also listed five installations at Brookhaven National Laboratory on Long Island, including a giant particle accelerator that runs through miles of tunnels. At the Livermore lab, the Russians were to get access to a $5 billion laser the size of a football stadium designed to ignite miniature hydrogen bomb explosions.
The September accord was posted online late last year by the Center for Public Integrity, a nonprofit news organization based in Washington. The disclosure received little public notice.
In October, a month after the accord was signed, Russian and American scientists from the nuclear laboratories held preliminary discussions on planetary defense, said an American official who spoke on the condition of anonymity because of the topic's political implications.
In January, the State Department held a space forum in Washington with representatives of many countries, including Russia. It called for international cooperation on projects such as defending Earth from asteroids.
"We may have different flags patched to our spacesuits," John P. Holdren, the president's science adviser, said in an opening address. But as cooperative space projects have demonstrated, he added, "we can transcend these differences."
William J. Burns, the deputy secretary of state, told the group that the United States welcomed global support for missions that would "help us learn how to better defend our planet from a catastrophic asteroid collision."
While asteroid defense may seem like the stuff of science fiction, the risk burst into public consciousness early last year when a meteor exploded over Russia and injured more than 1,200 people, mostly as windows shattered into clouds of flying glass. "It's not a laughing matter," said William E. Burrows, author of the new book "The Asteroid Threat." "If it brings the international community together, that's a good thing."
But the cooperative mood vanished after the invasion of Crimea. Russia complained bitterly: In April, Rosatom, its state nuclear energy company and partner in the accord, put out a statement calling the suspension of the partnership "a mistake that contradicts the constructive atmosphere that has built up."
Politics, it added, "should have no place in this field."