The U.S. Army's recent embrace of counterinsurgency warfare and nation building complicates theories of military politics. For decades, critics declared the army too risk averse, too parochial, and too insulated to change, often thwarting civilian demands for greater flexibility. How should we understand these recent, unexpected changes? In this article I synthesize insights from historical institutionalism and American political development to derive a micropolitical perspective on institutional change. This approach advances two components as necessary before an institution transforms. First, mid-level agents shift the unofficial discourses through which they understand and describe the institution's core missions and capabilities. These slow and often subtle changes create a mismatch between the mid-level actors and the institution's paradigm. This erosion of institutional order provides an opportunity to reformers. The second component of transformation is the work of these reformers to forge coalitions with elites inside and outside government and press institutional leaders for change. In the rest of the article, I demonstrate the efficacy of the micropolitical approach by investigating how the army developed its AirLand Battle doctrine after the Vietnam War. My analysis of recently declassified correspondence, oral-history interviews, and the writings of officers and experts shows how mid-level officers and external reformers were able to shift the discourses of army leaders and develop an institutional paradigm that endured for decades. Indeed, AirLand Battle influenced the Weinberger criteria for deploying American troops, and it shaped U.S. conduct during the Persian Gulf War of 1991. This suggests a research program that could demonstrate why and how the U.S. Army's way of war changed during the 2000s, as well as how durable this transformation will be.