Kennedy sought to understand the social, economic, and military forces that shape great powers. While earlier scholars of international history had written about 'great men' and their achievements, Kennedy focused on the interdependence of military might and economic growth. In this 1987 work, he shows why countries that balance economic and military investments can become 'great powers.' Those that fail to do so risk 'imperial overstretch,' and ultimately failure. Kennedy's ability to look back but also forward—he accurately predicted that the United States would face precisely this kind of challenge, which it did after 9/11—has earned him great respect.