For nearly two centuries, international relations have been premised on the idea of the "Great Powers." As the thinking went, these mighty states—the European empires of the nineteenth century, the United States and the USSR during the Cold War—were uniquely able to exert their influence on the world stage because of their overwhelming military capabilities. But this conception of power fails to capture the more complicated truth about how wars are fought and won.

Our focus on the importance of large, well-equipped armies and conclusive battles has obscured the foundational forces that underlie military victories and the actual mechanics of successful warfare.

Phillips O'Brien suggests a new framework of "full-spectrum powers," taking into account all of the diverse factors that make a state strong—from economic and technological might, to political stability, to the complex logistics needed to maintain forces in the field.

Drawing on examples ranging from Napoleon's France to today's ascendant China, he offers a critical new understanding of what makes a power truly great.

Phillips Payson O'Brien is a professor of strategic studies and head of the School of International Relations at the University of St. Andrews in Scotland. He is the author of six books, including his latest War and Power: Who Wins Wars—and Why.