TWENTY YEARS ON, it can be hard to recall the chaos and uncertainty that spread across the Soviet Union in the months after the superpower imploded in December 1991. The strict controls that had been a hallmark of the Soviet system seemed to vanish. A sprawling inventory of nuclear, chemical and biological weapons and materials lay vulnerable, often protected by no more than a padlock.

In Washington, there were sighs of relief at the end of the Cold War, but there was deep ambivalence about aiding a former adversary. Much to their credit, Sens. Sam Nunn (D-Ga.) and Richard G. Lugar (R-Ind.) rallied Congress to approve legislation that transferred $400 million in Pentagon spending to begin securing nuclear weapons in the former Soviet Union.

Nunn-Lugar, as it came to be known, was one of the most farsighted foreign policy initiatives of its generation. It helped dismantle a vast arsenal, from giant submarines carrying nuclear-armed missiles to chemical-weapons shells that could fit in a briefcase, and then expanded well beyond Russia. At a current budget of $1 billion a year in the departments of Defense, Energy and State, the program has been a bargain if you think about what might have happened without it.

It was never going to be easy for the United States to extend a hand to Russia in this way. Russia’s sense of humiliation has been a constant undercurrent. But many in Russia recognized the dangers and the country’s meager resources in those early post-Soviet years. They did the right thing, and cooperation paid off not only in scrapping warheads but also in alleviating mistrust, increasing transparency and deepening stability.

Today, Russia can afford to dismantle weapons on its own. The Foreign Ministry announced Wednesday that it will not extend the Nunn-Lugar agreement when it expires next year. No doubt, the decision is motivated in part by President Vladimir Putin’s desire to reassert Russia’s sense of self-sufficiency and his own primacy. He recently expelled the U.S. Agency for International Development, which was funding pro-democracy and civil society programs, and pushed legislation through parliament that would stigmatize as “foreign agents” nongovernmental organizations from abroad working in Russia. Alleviating mistrust is not his priority.

On weapons, the important question is not aid but willpower. Will Russia keep up the dismantlement effort on its own? Despite great strides, the legacy of the Cold War has not been completely secured nor cleaned up. Russia, like the United States, is also building new weapons. A wise next step would be to negotiate a replacement agreement that would better fit Russia’s revival, keeping both countries engaged and focused on the unfinished business. That might be difficult in the current political environment, but Nunn-Lugar stood the test of time over two decades, and it is too soon to give up on its mission.