Even during the Cold War things were never this bad, U.S. officials say. On 5 October, the Russian government suspended an agreement with the United States on nuclear R&D cooperation and terminated another on retooling Russian research reactors to no longer run on weapons-grade uranium fuel. The suspensions are largely symbolic, but have nonetheless plunged relations between the world’s most formidable nuclear powers to a new low and driven a new wedge between nuclear science communities that had forged close ties in the wake of the Soviet Union’s collapse a quarter-century ago.

In announcing the suspension of the R&D agreement, the Russian government framed it as a “countermeasure” to U.S. sanctions imposed on Russia after its annexation of Crimea in March 2014 and support for rebels in eastern Ukraine. The collapse of Syrian peace talks and sharp U.S. criticism of Russia’s involvement in the bombing of Aleppo, Syria, appear to have precipitated Russia’s delayed retaliation, sources say. Russia also pulled out of another agreement with the United States on 3 October in which the two countries were working to eliminate 34 tons of plutonium stockpiled in both countries—enough for about 17,000 bombs.

“We were really sorry to see the Russians do this,” says an official with the National Nuclear Security Administration (NNSA) of the U.S. Department of Energy (DOE) in Washington, D.C., who agreed to speak with Science on background. As the nations with the two biggest nuclear arsenals by far, “the United States and Russia have a special responsibility to work together,” adds Andrew Bieniawski, vice president for material security and minimization at the Nuclear Threat Initiative (NTI), a nonprofit based in Washington, D.C., that promotes nonproliferation.

Russia’s suspension of the nuclear R&D agreement, signed in September 2013, “is particularly unfortunate,” says the NNSA official, who calls it “the first framework following the collapse of the Soviet Union in which the two sides came together as equals.” After the Soviet Union fell apart in 1991, U.S. assistance to Russia’s nuclear establishment was geared toward securing nuclear materials and redirecting weapons scientists to civilian projects. Russian President Vladimir Putin long chafed at the United States’s helping hand, however. As Russia’s economy strengthened during the 2000s, Putin bolstered the nation’s impoverished nuclear complex; Rosatom, DOE’s Russian counterpart, then pushed for a more equitable R&D relationship with the United States. The terms of the agreement were negotiated in 6 months—lightning fast, the official says. “This was envisioned as a new model,” Bieniawski says.

The agreement called for wide-ranging cooperation in advanced power reactors and nuclear fuels, nuclear nonproliferation and safety, nuclear fusion research, and remediation of radioactive sites. In the fall of 2013, scientists from the U.S. national labs and counterparts in Russia’s nuclear complex began identifying potential projects and thrashing out technical considerations. For instance, after the March 2011 disaster at the Fukushima nuclear power plant in Japan, one area that seemed ripe for cooperation was developing accident-resistant nuclear fuel cladding.

U.S. sanctions on Russia’s moves against Ukraine in early 2014 chilled the budding research teamwork, however. DOE officials were still open to working with Russia on nonproliferation and nuclear security, and floated ideas for projects in these areas. “The Russians made it very clear that nonproliferation cooperation was not their priority,” the NNSA official says. Russia was keen on nuclear energy R&D, but the United States shied away because it was concerned that Russia would “use cooperation on nuclear energy as a political tool.”

Russia’s termination of the research reactor agreement may be less consequential, as Russia has shown little enthusiasm for converting its research reactors to use low-enriched uranium instead of weapons grade highly enriched uranium (HEU). “They did not see their own research reactors as a priority,” Bieniawski says. Yet Russia and the United States have worked well together in converting HEU reactors in other countries and repatriating HEU to Russia. Just last week, for instance, Russia and the United States completed the removal of 61 kilograms of HEU from the Maria research reactor in Poland—the last of about 700 kilograms of HEU, enough for more than two dozen bombs, that the two nations worked together for more than a decade to remove from Poland.

Russia is keeping the door open to a rapprochement. In suspending the research agreement, the Russian government stated that cooperation could resume “when that is justified by the general context of relations with the United States.” “There’s a huge amount of work that can be done,” Bieniawski says, noting that NTI has identified 51 projects that would benefit both countries. (NTI intends to release the list later this year.) For now, however, a barrier has been thrown up between the nations’ nuclear scientists. “At least during the Cold War,” the NNSA official remarks, “we had pretty good scientific exchanges.” Those days are long gone.