Nuclear security around the world has improved dramatically over the last three decades—which demonstrates that with focused leadership, major progress is possible. But important weaknesses remain, and the evolution of the threat remains unpredictable. The danger that terrorists could get and use a nuclear bomb, or sabotage a major nuclear facility, or spread dangerous radioactive material in a “dirty bomb,” remains too high. The United States and countries around the world need to join together and provide the leadership and resources needed to put global nuclear security on a sustained path of continuous improvement, in the never-ending search for excellence in performance.

With the world focused on the United States and North Korea, it’s easy to forget that every president for a quarter-century has said preventing nuclear terrorism was a national security priority. This includes the Trump administration, which identified in its Nuclear Posture Review that nuclear terrorism is one of “the most significant threats to the security of the United States.” It appears, however, despite this strong rhetoric, the administration may not be putting its money where its mouth is.

The escalating threats between North Korea and the United States make it easy to forget the “nuclear nightmare,” as former US Secretary of Defense William J. Perry put it, that could result even from the use of just a single terrorist nuclear bomb in the heart of a major city.

At the risk of repeating the vast literature on the tragedies of Hiroshima and Nagasaki—and the substantial literature surrounding nuclear tests and simulations since then—we attempt to spell out here the likely consequences of the explosion of a single terrorist nuclear bomb on a major city, and its subsequent ripple effects on the rest of the planet. Depending on where and when it was detonated, the blast, fire, initial radiation, and long-term radioactive fallout from such a bomb could leave the heart of a major city a smoldering radioactive ruin, killing tens or hundreds of thousands of people and wounding hundreds of thousands more. Vast areas would have to be evacuated and might be uninhabitable for years. Economic, political, and social aftershocks would ripple throughout the world. A single terrorist nuclear bomb would change history. The country attacked—and the world—would never be the same.

During the latter part of the Cold War, many strategists thought of nuclear deterrence and arms control as two of the most essential stabilizing elements of the same strategy in managing an adversarial relationship. The renewed crisis between the West (the United States and NATO member states) and Russia demonstrates how critical these elements are to the strategic nuclear relationship. As a result of recent setbacks between Washington and Moscow in the past few years, arms control has taken a back seat, and the risk of conflict due to miscalculation is the highest it has been since the 1980s.

Nickolas Roth reflects upon Doomed to Cooperate by Siegfried S. Hecker, which tells the story of how, after the Cold War ended, U.S. and Russian scientists worked together to strengthen Russian nuclear safety and security, reduce proliferation risks, and advance nuclear science. He identifies that the book provides important lessons for policymakers in each country who are, just as they were more than two decades ago, scrambling to cope with the rapidly changing relationship between the world’s two largest nuclear superpowers.

This spring, the United States hosted the fourth and final Nuclear Security Summit in Washington, DC. Senior representatives of more than 50 nations convened to mark the end of an unprecedented international initiative over the last six years to strengthen security measures aimed at preventing nuclear terrorism. During that time, many states made significant progress, but more work is needed.

"Hillary Clinton and Donald Trump are once again trading barbs over who has the right disposition to be president -- including who is best suited to have a "finger on the button. It is, of course, not unusual for candidates to critique each other's qualifications. But there is a good reason why assessing the candidates' judgment and temperament is particularly important: The president of the United States has complete authority over more than 1,000 nuclear weapons ready to launch...."

The 2016 nuclear security summit was a pivotal moment for the decades-long effort to secure nuclear material around the globe. More than 50 national leaders gathered in Washington for the last of four biennial meetings that have led to significant progress in strengthening measures to reduce the risk of nuclear theft.