With the world focused on the nuclear crisis in Iran, it is tempting to think that addressing this case, North Korea, and the problem of nuclear terrorism is all that matters and is what matters most. Perhaps, but if states become more willing to use their nuclear weapons to achieve military advantage, the problem of proliferation will become much more unwieldy. In this case, U.S. security will be hostage not just to North Korea, Iran, or terrorists, but to nuclear proliferation more generally, diplomatic miscalculations, and wars between a much larger number of possible players. This, in a nutshell, is the premise of Underestimated: Our Not So Peaceful Nuclear Future, which explores what nuclear futures we may face over the next 3 decades and how we currently think about this future. Will nuclear weapons spread in the next 20 years to more nations than just North Korea and possibly Iran? How great will the consequences be? What can be done?