To the Editor:

Niall Ferguson ("Complexity and Collapse," March/April 2010) is correct to caution that excessive debt could weaken the United States if it persists over a long period, but he is unconvincing in using complexity theory to warn of a sudden decline.

Ferguson argues that "most imperial falls are associated with fiscal crises," but his own evidence suggests otherwise. The swiftness of the decline of the Hapsburg, Ottoman, Romanov, and German empires was caused by war. Moreover, the end of a regime -- the Ming dynasty, for example -- is not always the end of a state's power. And it seems odd to describe the Western Roman Empire's decline as rapid when three centuries passed between its apogee and its demise.

The United States faces a number of problems (including debt) that may hinder its response to what the author Fareed Zakaria has called "the rise of the rest." But sudden collapse seems one of the less likely scenarios.

University Distinguished Service Professor and Sultan of Oman Professor of International Relations, Harvard University