President Barack Obama's upcoming Nuclear Security Summit has the potential to become a defining moment for international security in the twenty-first century, especially after the recent release of the U.S. Nuclear Posture Review. When he introduced this document, Obama said, "For the first time, preventing nuclear proliferation and nuclear terrorism is at the top of America's nuclear agenda."

Two years ago in Prague, President Barack Obama laid out his vision of a world free of nuclear weapons. Recognizing that this would not likely be achieved in his lifetime, he outlined practical steps by which the international community might strive for greater security in a world where nuclear weapons still exist. One of these steps was the four-year goal to secure all loose nuclear material -- almost immediately after his speech, concerns about nuclear terrorism and nuclear material security were prominent on the international agenda.

Is a terrorist nuclear attack still something to worry about, 20 years after 9-11? The short answer, unfortunately, is “yes.” The probability of terrorists getting and using a nuclear bomb seems low—but the economic, political, and social consequences would reverberate far and wide—and change history.

The attack and occupation of nuclear reactors by Russia—a nuclear weapon state—exposed a wholly new kind of risk, which needs a new term.

Terrorists could use a nuclear bomb in ways that don’t include detonating it. By planning for a range of scenarios, policymakers can prevent the worst outcomes from happening.

Without sufficient international cooperation Biden’s new nuclear security agenda is unlikely to succeed, a nuclear security expert argues.

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The world relies too much on the indefinite continuation of the post-1945 taboo on military use of nuclear weapons.

The text of an address given at the Asahi Shimbun Symposium in Nagasaki, Japan, late in July.