If you ask Americans to identify the greatest risk to our national security, most will probably point to terrorism. That's understandable in the wake of the horrific events in Orlando and San Bernardino, as well as incidents around the world, such as Thursday's Bastille Day attack in Nice, France.
Others will say the greatest risk comes from climate change or illegal immigration or potential conflict with China or Russia. All of these are urgent problems that must be addressed.
From my point of view, however, the gravest threat to our security and well-being as a nation is the threat posed by nuclear weapons. It may not be the most likely threat to come to pass, but it is the most consequential threat we face.
Even a single nuclear detonation could kill tens of thousands of people and massively disrupt our way of life. In the words of former Secretary of Defense William Perry, a prominent expert on nuclear proliferation: A nuclear conflict could bring an end to our civilization.
And this is to a great extent a hidden threat. There is little public awareness or concern about the issue, and it is rarely discussed in the news media.
It is time for our leaders to accelerate their efforts to deal with this problem. But they are not likely to do so unless the American public understands the danger and demands action.
During the Cold War, we relied on the doctrine of "mutually assured destruction." We believed that neither the United States nor the Soviet Union would launch an attack because it would produce massive and destructive retaliation. But the world has changed, and we are in a high-risk era when the presence of nuclear weapons raises the stakes for global conflict, accidents and terrorism.
Nine nations possess nuclear arms, and we don't have a reliable system to track and account for weapons-useable nuclear materials that are present in 24 countries. Several of these countries lack the capacity to control them or fail to put the highest priority on their security.
Terrorists clearly see these materials as a target. ISIS and other extremist groups could gain the capacity to detonate a "dirty bomb" that would use conventional explosives to spread radioactive material over a wide area. There isn't any doubt that, if terrorists got these weapons, they would use them.
Two countries, the United States and Russia, have a special responsibility to lessen the risk of nuclear weapons, simply because we created, deployed and still possess the great bulk of these weapons. But to do that, we must have sustained cooperation and diplomacy, and we don't. Dialogue is largely absent. Tensions are on the rise, and both countries are pursuing a new generation of smaller nuclear weapons.
Some experts say the risk of a nuclear catastrophe is greater now than it was during the Cold War. While our goal should be a world without nuclear weapons, that won't happen soon. We need to act now.
First, we need to increases the time allowed for leaders to decide whether to launch a nuclear missile response to another country's attack. We are operating on an outdated "launch-on-warning" system that serves no strategic purpose and increases the risk of launching an attack by mistake.
We also need to step up our efforts to reduce the quantity of nuclear weapons and materials. We need stronger and more transparent systems for verifying these reductions. Without verification, we will not be able to persuade nations to comply with international agreements.
And we must become much more effective, along with the rest of the international community, at securing nuclear materials and keeping them out of the hands of terrorists.
There are obstacles to progress, including economic interests tied to producing and maintaining nuclear weapons, the sheer complexity of how to achieve a reduction and the nationalistic pride that drives nations to develop and hang onto their weapons. But there is broad international support for taking the steps that are needed. And we have strong leadership on the issue from outside government, including Secretary Perry, former Secretaries of State George Schultz and Henry Kissinger and former Sen. Sam Nunn, who have written a series of articles outlining the threat of nuclear weapons.
President Barack Obama argued for ridding the world of nuclear weapons in the first major foreign policy speech of his presidency, delivered in Prague in April 2009. And some progress has been made, including adoption by the U.S. and Russia of the New START treaty that limits the number of strategic nuclear weapons for each side and provides new means of verification.
The effort has waned, however, as terrorism and the conflicts in the Middle East have demanded immediate attention. Recent reports indicate that President Obama plans to take steps before he leaves office to advance his Prague agenda. But real progress will require more than executive action.
Gen. Omar Bradley, the distinguished commander of American troops in World War II, told us that the only way to win an atomic war is to ensure that it never starts. We are fortunate to have avoided a nuclear disaster for seven decades. But luck is not a strategy, and we cannot afford to rely on it.
The world is filled with dangers, but nuclear weapons constitute a transcendental threat, capable of putting civilization at risk. We must not be complacent in the face of this threat or give up. We must acknowledge it and take steps to back away from the nuclear brink.
Lee H. Hamilton is a Distinguished Scholar, Indiana University School of Global and International Studies; Professor of Practice, IU School of Public and Environmental Affairs; and Senior Advisor, IU Center on Representative Government. He served as U.S. Representative from Indiana's 9th Congressional District from 1965-1999.