Today between forty and sixty nations, home to more than one billion people, have either collapsed or are teetering on the brink of failure. The world's worst problems--terrorism, drugs and human trafficking, absolute poverty, ethnic conflict, disease, genocide--originate in such states, and the international community has devoted billions of dollars to solving the problem. Yet by and large the effort has not succeeded.

Ashraf Ghani and Clare Lockhart have taken an active part in the effort to save failed states for many years, serving as World Bank officials, as advisers to the UN, and...