Why does the United States struggle to build stronger militaries in partner states? The fundamental challenge of security assistance is that of influencing recipient political-military decision-making. How does the United States aim to influence recipient leaders? Which strategies of influence work best? Why does the United States choose the strategies of influence it chooses? I conceptualize U.S. influence strategies in security assistance as an influence escalation ladder with four rungs: teaching, persuasion, bargaining, and direct command. I develop Influence Strategy Theory (IST), arguing that the United States is more likely to successfully influence partners and build better partner militaries when it employs the full escalation ladder. It is less likely to succeed when it relies exclusively on teaching and persuasion. Moving a link back in the causal chain, I offer two competing models of strategy selection—the rational actor model, and the Cult of the Persuasive. I argue that the rational actor model sufficiently explains U.S. strategy in pre-Vietnam security assistance efforts, but cannot explain U.S. advisors’ persistent reliance on persuasion in Vietnam and thereafter. In Vietnam, the U.S. Army untethered from its civilian principal in Washington to instead pursue its parochial bureaucratic interests. An institutional ideology—“the cult of the persuasive”— preaching the normative and causal superiority of persuasion over coercion evolved within the U.S. Army to minimize disruption of its bureaucratic machinery. The ideology continues to guide U.S. security assistance today because the U.S. military has no institutional incentive to change course. I test these arguments within and across three critical cases of U.S. security assistance, with chapters examining the U.S. effort to build the Republic of Korea Army (1948 – 1953), the Army of the Republic of Vietnam (1955 – 1973), and the Iraqi Army (2003 – 2011). I draw from thousands of archival documents, over 500 oral histories collected from former U.S. advisors, and over 150 original interviews. I find strong support for the expectations of the study. The findings provide new theoretical and empirical insights for students of security assistance and military strategy, as well as practical lessons for policymakers and military advisors.Ph.D.