America is now threatened less by conquering states than we are by failing ones.” That was the conclusion of the 2002 U.S. National Security Strategy. For a country whose foreign policy in the 20th century was dominated by the struggles against powerful states such as Germany, Japan, and the Soviet Union, the U.S. assessment is striking. Nor is the United States alone in diagnosing the problem. U.N. Secretary-General Kofi Annan has warned that “ignoring failed states creates problems that sometimes come back to bite us.” French President Jacques Chirac has spoken of “the threat that failed states carry for the world’s equilibrium.” World leaders once worried about who was amassing power; now they worry about the absence of it.

Failed states have made a remarkable odyssey from the periphery to the very center of global politics. During the Cold War, state failure was seen through the prism of superpower conflict and was rarely addressed as a danger in its own right. In the 1990s, “failed states” fell largely into the province of humanitarians and human rights activists, although they did begin to consume the attention of the world’s sole superpower, which led interventions in Somalia, Haiti, Bosnia, and Kosovo. For so-called foreign-policy realists, however, these states and the problems they posed were a distraction from weightier issues of geopolitics.

Now, it seems, everybody cares. The dangerous exports of failed states—whether international terrorists, drug barons, or weapons arsenals—are the subject of endless discussion and concern. For all the newfound attention, however, there is...