(FILES) This file picture dated 1998 shows a Russian Army missile launch pad during a display of the replacement of a nuclear missile by another at the Tamanskaya division, Saratov region. (-/AFP)

Sam Nunn is co-chairman of the Nuclear Threat Initiative and a former U.S. senator from Georgia. Richard Lugar is president of the Lugar Center and a former U.S. senator from Indiana.

For more than two decades, the United States and Russia partnered to secure and eliminate dangerous nuclear materials — not as a favor to one another but as a common-sense commitment, born of mutual self-interest, to prevent catastrophic nuclear terrorism. The world’s two largest nuclear powers repeatedly set aside their political differences to cooperate on nuclear security to ensure that terrorists would not be able to detonate a nuclear bomb in New York, Moscow, Paris, Tel Aviv or elsewhere.

Unfortunately, this common-sense cooperation has become the latest casualty of the spiraling crisis in relations among the United States, Europe and Russia.

In December, Congress voted — for the first time in nearly a quarter-century — to defund U.S. efforts to secure vulnerable nuclear materials in the Russian Federation. Days later, Russian officials, following up on previous signals, informed their U.S. counterparts that Russia was cutting off most aspects of its nuclear security cooperation with the United States. These shortsighted actions send a dangerous message to the international community and represent a major setback in the global effort to secure nuclear materials.

Given the standoff over Ukraine, it is inevitable that many elements of the U.S.-Russian bilateral relationship will come under severe strain. However, the United States and Russia share a fundamental interest in nuclear security and, with it, a special responsibility to cooperate in this realm. Both countries have been victimized by terrorism, and both continue to be targets of terrorist organizations. A terrorist attack involving a nuclear weapon in any country would deal a devastating blow to global security, the global economy and our way of life. That is why American and Russian leaders cannot allow contention on other fronts to prevent them from pursuing mutually beneficial steps on nuclear security to avoid a disaster.

We have a strong history upon which to build. Cooperation between Washington and Moscow to secure or eliminate weapons of mass destruction dates to 1991, when the Soviet Union was collapsing and the security of its vast nuclear arsenal was in serious doubt. To prevent a nuclear catastrophe, the two of us worked with a bipartisan group of senators to pass legislation that formed the basis of what became known as the Nunn-Lugar Cooperative Threat Reduction Program. Under this program, the United States provided assistance to Russia and the other countries of the former Soviet Union to deactivate more than 7,600 nuclear warheads, eliminate more than 4,100 metric tons of chemical weapons, destroy more than 2,600 nuclear delivery vehicles and secure dozens of Soviet-era weapons facilities.

While these efforts dramatically reduced the dangers posed by the legacy of the Cold War, the work to secure Russia’s nuclear complex and to secure dangerous materials globally is far from over. Just last August, a high-level Energy Department advisory panel concluded, “Russia continues to have the world’s largest stockpiles of nuclear weapons, separated plutonium, and highly enriched uranium (HEU), in the world’s largest number of buildings and bunkers — and a variety of vulnerabilities remain that a sophisticated conspiracy could exploit.”

Unfortunately, the United States is no stranger to nuclear security vulnerabilities. We have experienced a number of security breaches at our nuclear sites, including an incident two years ago, when an 82-year-old nun and two others staging a protest managed to break into one of the world’s most secure nuclear facilities in Oak Ridge, Tenn. If an unarmed nun is capable of breaking into America’s nuclear Fort Knox, we must entertain the possibility that terrorists could do the same, with much more serious consequences. Indeed, incidents like these underscore the need for global cooperation to enhance the security of nuclear material, much of which is housed in far less secure locations than Oak Ridge.

This effort must be global because the threat is global. In 1992, 50 countries possessed weapons-usable nuclear materials. Today, that number has been halved, thanks in no small part to bilateral cooperation between the United States and Russia.

Cooperation, however, need not be dominated by unilateral U.S. assistance. We need a new approach — a real nuclear security partnership guided by the principles of reciprocity and mutual interest, to which both countries contribute their own funding and technical resources.

Such a partnership should include: accelerating efforts to repatriate and eliminate U.S. and Russian-origin highly enriched uranium from other countries; collaborating on research and development of innovative nuclear security technologies; expanding nuclear security best-practice exchanges; and utilizing the extensive U.S. and Russian technical expertise to help support nuclear security improvements in other countries with nuclear materials.

These steps and others could be achieved on the basis of mutual interest without major concessions from either side. This will be impossible, however, if cooperation to prevent catastrophic terrorism is regarded as a geopolitical bargaining chip. Failing to cooperate in this area is a “lose-lose” proposition that would damage the vital interests of both nations and vastly increase the risk of nuclear terrorism. The United States and Russia must recognize the imperative to provide global leadership. The consequences of inaction are simply too great.