Nuclear terrorism is the scariest security threat there is: it involves all of the destructive power of nuclear-armed states with none of the checks. Deterrence has seen us through in the past, but the concept, it is argued, is moot for a small group with a suicide mission mentality. Worst case scenarios abound as experts try to forecast how such a situation would play out.

Michael Levi's new book, On Nuclear Terrorism, is meant to help us "kick the habit" of forecasting doomsday scenarios and demanding perfect defenses. Writing on TPM Cafe, Levi argues that "No Defense Is Perfect," but luckily there are so many places for a nuclear terrorist to fail along the way that we have many more opportunities for disrupting such plots than we might think.

In 1984, the IRA came within a whisker of killing Margaret Thatcher in her hotel room at Brighton's Grand Hotel. The day after, it issued a statement: "Today we were unlucky, but remember; we only have to be lucky once. You will have to be lucky always." That maxim has become popular amongst our political leaders in recent years.

If you look inside nuclear terrorism, though, the exact opposite becomes true. Move from stage to stage of a nuclear plot and you will discover a host of ways that a terrorist group might fail. That turns the old saw on its head. We only have to be lucky once for a plot to fail. To succeed, a terrorist group must be lucky always.

Levi takes this threat seriously, but he doesn't want the issue to succumb to the crushing weight of impossible odds that can plague scary global issues. He breaks the problem down into more manageable pieces, which is usually the first step toward constructive action.