Thanks to disputes over Ukraine, Syria, and elsewhere, the United States and Russia — the two countries that hold the vast majority of worldwide fissile material — have ceased nearly all cooperation that could thwart a nuclear terror attack.

Such a scenario is not farfetched. There is documented evidence that al-Qaeda and other terrorist groups have pursued weapons-grade nuclear materials, and smugglers have been caught attempting to distribute them to the wrong hands. Just 25 kilograms of highly enriched uranium could fuel a crude, 15-kiloton nuclear weapon. If al-Qaeda had detonated such a device in lower Manhattan instead of flying airplanes into towers, the toll in history’s deadliest terror attack would have been not 3,000 souls but forty times as many.

The current U.S.-Russia relationship is plagued by big and difficult issues. They include, but are not limited to, Russia’s violations of the Intermediate-Range Nuclear Forces Treaty, alleged interference in the U.S. presidential election, annexation of Crimea, attempts to undermine the government of Ukraine, and military actions in Syria.

Despite these differences, there are still some areas of cooperation between the two countries. For example, NASA pays the Russian space program to give American astronauts a ride to the International Space Station while the U.S. develops its next-generation space capabilities. The countries are also working together to enforce the Joint Comprehensive Plan of Action, also known as the Iran nuclear deal.

Thwarting nuclear terrorism should be added to the list. The risk of a nuclear terror attack in Washington or Moscow — or anywhere on the planet — cannot be ignored due to other disagreements.

It would take only one nuclear terror attack to radically alter our way of life.

Such cooperation has a track record of success. In 1991, as the Soviet Union was nearing its dissolution, global concern focused on an unthinkable danger: the soon-to-be-independent Ukraine, Belarus, and Kazakhstan stood to inherit Soviet nuclear weapons, along with fissile material that could be used to make more nuclear arms. In response, my former colleagues, senators Sam Nunn and Richard Lugar, passed an amendment to provide funding for these new states — and Russia itself — to help secure and remove the missiles, warheads, and material.

What became known as Nunn-Lugar Cooperative Threat Reduction (CTR) was a resounding success. Because of U.S. assistance and joint U.S.-Russian implementation, about 7,600 warheads have been deactivated and 2,500 missiles destroyed, and dozens of nuclear weapons storage facilities are more secure. Some former Soviet missile silos, which once housed nuclear warheads pointed at the United States, are now fields covered by sunflowers. Ukraine, Belarus, and Kazakhstan are non-nuclear weapons states, and the world is a safer place because of it.

Yet the work was far from done when Russia notified the United States that it would not renew the CTR agreement after it expired in 2013. Shortly thereafter, Moscow announced that it was declining any further U.S. funding to improve security at Russian sites containing weapons-usable nuclear material. In 2016, Russia suspended agreements that worked to convert its research reactors from using highly enriched uranium — which could be used by a terrorist group to create an improvised nuclear device — to safer low-enriched uranium. Russian President Vladimir Putin also walked away from another agreement that committed both countries to reducing their excess stocks of weapons-grade plutonium. And the Kremlin’s delegation skipped the fourth and likely final Nuclear Security Summit in 2016, an American-led initiative to help secure all nuclear materials on the planet.

When the United States and Russia fail to work together to reduce the threat of nuclear weapons and proliferation, it jeopardizes our entire planet.

But Russia’s intransigence is not the only problem. Some U.S. lawmakers believe that nuclear security cooperation with Moscow has become unnecessary, and Congress has imposed needless hurdles to it. Furthermore, in response to Russia’s annexation of Crimea and its activity in Eastern Ukraine, the U.S. Department of Energy unilaterally halted nearly all American-Russian nuclear energy cooperation, including a joint commission on nuclear energy and security.

Leaders in both countries must recognize that when the United States and Russia fail to work together to reduce the threat of nuclear weapons and proliferation, it jeopardizes our entire planet. Other disagreements notwithstanding, the two countries must restart their joint efforts to secure dangerous nuclear materials.

It would take only one nuclear terror attack to radically alter our way of life. Should this happen, the public would wonder why alleged treaty violations, military differences, and geopolitical disagreements prevented action to avert catastrophe.