From the time of their creation in 1945, until relatively recently, nuclear weapons were the province of great powers. By definition and as a matter of practical reality, states that possessed nuclear weapons were great powers, and only great powers could marshal the resources necessary to attain them.

Several forces, however, are now converging to undermine not only the great power monopoly on nuclear weapons, but the very idea that only nation-states can acquire such devastating power. That Al Qaeda seeks to acquire nuclear weapons and would not hesitate to use them is, unfortunately, a 21^st century reality. Al Qaeda, however, is not the only terrorist group to have sought nuclear weapons. The apocalyptic cult Aum Shinrikyo contemplated acquiring nuclear weapons before launching a deadly nerve gas attack in Tokyo's subway in 1995.

A key to preventing the spread of nuclear weapons is controlling the fissile materials-Highly Enriched Uranium and plutonium-which form the core of nuclear weapons. If terrorists or rogue states are to acquire nuclear weapons, they must make, steal, or buy such material. Each of these disparate paths entails different challenges.