To reduce the risk of nuclear terrorism, we must prevent terrorists from obtaining nuclear weapons or materials. This will require, among other things, a sustained effort to keep dangerous nations from going nuclear--in particular North Korea. This article reviews the efforts the United States has undertaken through the years to keep North Korea from building a nuclear arsenal, arguing that the history of proliferation on the Korean Peninsula is marked by five nuclear crises. A sixth could be on the horizon, further compromising American efforts to lessen the likelihood of a nuclear attack on U.S. soil.