Conflict in the twenty-first century involves a complex interaction of variables. Equally complex are modern stability operations that occur concurrently or immediately following war or combat operations. This study examines planning methodology for stability operations, focusing on the specific doctrinal planning construct of "lines of operation", one of the "Elements of Operational Design" contained in the US Army's Field Manual 3-0, Operations . The monograph first traces the historiography of the term "lines of operations" and examines its development from an essential geographical principle of linear warfare to a cognitive tool for modern planners. Included in this analysis is an evaluation of military operational theory, general systems theory, emerging ideas associated with complexity theory, and problem solving constructs based on nonlinear principles.