Why has the United States assumed so extensive and costly a role in world affairs over the last hundred years? The two most common answers to this question are "because it could" and "because it had to." Neither answer will do, according to this challenging re-assessment of the way that America came to assume its global role. The country's vast economic resources gave it the capacity to exercise great influence abroad, but Americans were long reluctant to meet the costs of wielding that power. Neither the country's safety from foreign attack nor its economic well-being required the...