More than 75 years after the end of World War II, military interventions—rather than major wars—have emerged as a defining feature of contemporary geopolitics. Yet, for all the fierce policy debates over interventions and their lessons, scholars have largely ignored the systematic linkages between these smaller-scale wars and transformations in the grand strategies of states that prosecute them. This book develops a new theory—the informational theory of strategic adjustment—to explain why military interventions can be crucibles of grand strategy. It argues that, by prosecuting a military intervention, states glean rich and rare information about adversaries’ capabilities and intentions, as well as their own military power and cost tolerance. The uniquely costly nature of warfighting renders this data particularly credible. Amidst background conditions of intense interstate competition and pervasive uncertainty, states face strong incentives to reassess their grand strategies in light of this new information. This process of grand strategic updating begins with a reassessment of the strategic assumptions directly tested on the battlefield, but it doesn’t end there. Indeed, the grand strategic effects of military interventions are far-reaching because information conveyed via warfighting is widely extrapolated to related strategic assessments. This book demonstrates the plausibility of the informational theory of strategic adjustment in three historically detailed case studies that trace the evolution of American grand strategy over the course of the Cold War and into the early post–Cold War era: the Korean, Vietnam, and First Gulf Wars.