Russian President Vladimir Putin attends the opening session of the newly elected State Duma, Russia's lower house of parliament, in Moscow, on Oct. 5. (Alexei Nikolsky/Pool photo via Associated Press)

THE IMPROVEMENT of security for nuclear materials like weapons-grade uranium and plutonium, as well as reductions in nuclear weapons arsenals, brought Russia and the United States together over the past 25 years. But the common ground has been disappearing as President Vladi­mir Putin takes a more confrontational stance toward the United States and tensions deepen over Russian actions in Syria and Ukraine.

When it was signed in 1987, the Intermediate-Range Nuclear Forces treaty was the first to wipe out a whole class of nuclear arms. Russia inherited the obligations of the Soviet Union to eliminate ground-based ballistic and cruise missiles with a range between about 300 and 3,400 miles. But the United States says Russia is quietly violating the treaty with a new ground-launched cruise missile, and recent reports suggest Russia may be making more of them. Russia has denied being in violation. The Obama administration has now asked for a rare meeting of a special commission established to hash out compliance issues. A violation of this magnitude, if true, cannot be overlooked; a failure to settle the issue could weaken the treaty.

Earlier, Mr. Putin announced the cancellation of an agreement that called for each nation to dispose of at least 34 metric tons of plutonium made surplus by the reduction in nuclear arsenals, all told enough for about 17,000 nuclear weapons. The point of the agreement was to show the rest of the world that the drawdown would be permanent and to keep fissile material from falling into the wrong hands. It was just one aspect of a much wider effort to secure nuclear, chemical and biological weapons materials in the former Soviet Union under legislation sponsored by then-Sens. Sam Nunn (D-Ga.) and Richard Lugar (R-Ind.).

The U.S. plan to mix the plutonium with uranium and turn it into fuel for power reactors began to falter some time ago. Construction began on a facility at the Energy Department’s Savannah River Site in South Carolina, but costs skyrocketed. The administration and outside experts found it would be a lot less expensive and faster to dispose of the plutonium by dilution and burial. Russia also tried mixing plutonium with uranium but now is heading toward another approach, using the plutonium as fuel for prototype fast-breeder reactors. Both nations are now free to go their own way, but lose the certainty of a joint pact. Russia says it won’t put the excess plutonium back into weapons work, but who is to know if it does?

Mr. Putin made a crude attempt to turn nuclear security into a bargaining chip. He complained about “unfriendly actions” by the United States, and demanded an end to all sanctions against Russia over the war in Ukraine; compensation for the damage they caused; repeal of the Magnitsky Act, which penalizes Russian officials involved in human rights violations; and pullback of U.S. forces from the new members of NATO. This is a misguided gambit. The nuclear security agreements were not created as a favor to the United States; rather they made the world a bit safer and helped avoid a potential catastrophe. Hardly the stuff of games.