Presidents appear imperial, but this is a carefully cultivated facade. While presidents publicly proclaim broad powers, their actions tell a different story. Although presidents have the ability to justify acting unilaterally in virtually any use of force, the executive is constrained by exposure to Loss Responsibility Costs: political costs suffered for an intervention that ends poorly.  American leaders will only risk full-scale war if they have the political cover provided by formal authorization, which simultaneously forces lawmakers to assume responsibility for the intervention. Presidents frequently launch smaller military actions unilaterally, in contrast, not because they are out of line with congressional preferences, but because having the president act unilaterally is preferred by lawmakers. When acting alone, presidents are, indeed, acting at their own peril: vulnerability to Loss Responsibility Costs constrains them from straying far beyond what congressional sentiment supports. Altogether, these dynamics imply that presidents are far more restrained by Congress than is usually supposed. 

This constraint, furthermore, has significant implications internationally: third parties naturally judge the American executive as less likely to use military force if they perceive the president facing considerable domestic obstacles to such action. Absent a clear signal of congressional support, threats and commitments made by an executive seen as restrained by Congress are less credible than those made by an unconstrained leader. In this way, perceptions of a constrained president can directly undermine the deterrence of U.S. adversaries and the reassurance of U.S. allies and partners.  Because of this, presidents face strong incentives to feign imperialism—to create a veneer of an imperial presidency in order to deter challenges and comfort friends. 

This book provides novel archival material and a unique dataset of congressional sentiment toward military force. The dataset, derived from both human and AI labeling of congressional floor speeches, spans approximately two hundred crises from the end of World War II to early 2022. It reveals that congressional sentiment has a much greater impact on the use of force than previously recognized. The work also draws on extensive archival material from U.S. presidential libraries, the Foreign Relations of the United States, the Digital National Security Archive, and foreign language sources, including a focused study on Chinese and Vietnamese perceptions of American credibility during the Vietnam War.