NEAR THE Russian city of Ozersk, about 80 miles south of Yekaterinburg, stands a warehouse like no other. The Fissile Material Storage Facility, with walls 23 feet thick, is a hanger-sized vault to protect fissile materials — highly enriched uranium and plutonium — that could be used for nuclear weapons. The facility was built at a cost of $309 million by the United States in a period of cooperation with Russia to prevent the spread of nuclear weapons materials.

Today, that period is ending. Russia informed U.S. officials in December that it wants to end nuclear security cooperation with the United States. Although anticipated for some time, the decision marks an unfortunate conclusion to an effort that was remarkable in a number of ways.

Created by Congress in 1991 as the Soviet Union was falling apart and championed by then-Sens. Sam Nunn (D-Ga.) and Richard Lugar (R-Ind.), the program extended the hand of a prosperous United States to Russia and other nations that emerged from the Soviet implosion with barely the shirts on their backs but truckloads of fissile material vulnerable to theft and diversion.

Despite decades of mistrust during the Cold War, the Nunn-Lugar program involved cooperation on the most sensitive of projects. Everything from intercontinental ballistic missiles capable of carrying nuclear warheads to artillery shells filled with deadly chemicals was destroyed and security upgrades were carried out at dozens of facilities. It wasn’t charity; the work had security benefits for both nations and the rest of the world. Overall, the program reflected a sense of magnanimity, bipartisanship and purpose in U.S. foreign policy.

After 20 years, Russia has become prosperous enough to pay for its own nuclear security, although it now faces new economic woes. In terminating nuclear cooperation amid the deepening tensions over Russia’s aggression in Ukraine, President Vladimir Putin is peevishly shooting his nation in the foot. Russia confronts serious terrorism threats, and its nuclear security, while improved, is not stellar. Fissile material remains spread out over too many time zones in too many different warehouses. On the opposite page today, Mssrs. Nunn and Lugar call for a “new approach” reflecting a more equal partnership. This is a reasonable goal, but the reality is that the bilateral relationship has soured over Mr. Putin’s perfidy and belligerence, creating a new gulf of mistrust.

Importantly, Nunn-Lugar is no longer just about Russia and the former Soviet Union. The programs have been used to get dangerous materials out of Libya and to destroy much of the Syrian chemical weapons stocks; to establish sentinels for biological threats on several continents; and to secure potentially vulnerable civilian nuclear material around the world, among other things. There is no question that the $1.6 billion a year or so spent for this goal has been a bargain. Two decades of operational experience in Nunn-Lugar may also prove valuable should another nuclear-armed nation suddenly collapse. Russia may be withdrawing, but the potential threats aren’t going away.