Beginning in 1999, the Army pursued a transformation effort that would span over a decade and produce a changed force structure that relied upon the brigade combat team as the service's focal conventional fighting force. Two decisions loomed large in the Army's direction away from the division as its combat force building block. This thesis examines both the decision to create the Stryker Brigade Combat Team as part of General Eric Shinseki's vision for Army transformation, and General Peter Schoomaker's decision in 2003 to focus change on the creation of a modular force. These decisions are investigated through three hypotheses that are based in military innovation theory. The hypotheses contend that the Army's decisions can be explained by either change in the security environment, by intervention on the behalf of civilian leaders external to the service demanding change, or by innovative thinking and leadership by the Army's senior uniformed or civilian leaders. This thesis finds that elements of each hypothesis were present in each decision, but that the impact of the security environment appeared as a strong causal factor in the Army's movement toward modularization throughout the examination of the entire time period.