The Combat Studies Institute (CSI) is pleased to present Long War Occasional Paper 27, "The Challenge of Adaptation: The US Army in the Aftermath of Conflict, 1953-2000," by CSI historian Mr. Robert Davis. Using three case studies from the late twentieth century, Davis examines the processes by which the US Army sought to prepare itself for the future after the conclusion of a major conflict. It is essentially a study of how, in the wake of major conflict, the Army "learned its lessons." In each of these periods - post Korean War, post Vietnam War, and post Cold War - the Army examined its existing institutional structures and processes, force structure, training and educational systems, and doctrine to prepare for an uncertain future. Following the Korean War, the nation struggled to define the role of ground forces in a Cold War era seemingly dominated by airpower and nuclear weapons. The Army also wrestled with the conceptual problem of creating a "dual-capable" force which could fight on both nuclear and conventional battlefields. President Kennedy's "Flexible Response" defense strategy and the Vietnam War abruptly ended the Army's unsatisfactory Pentomic Era. By contrast, after the Vietnam War the nation and the Army re-emphasized a "threat based" approach to developing and measuring its capabilities against the clearly defined military threats posed by the USSR and the Warsaw Pact. During the seventeen years between 1973 and 1990, the Army implemented wide-ranging institutional, doctrinal, training, educational and force structure changes which yielded an Army of unprecedented capability by the first Gulf War in 1991. After the end of the Cold War, however, the nation and the Army faced a very uncertain national security situation without a clearly definable threat. Like the advent of the nuclear era some fifty years prior, the information age seemed to call into question the role of ground forces in future military operations. Following a decade of unconventional operations in the 1990s, the Army launched another wide-ranging transformation effort in 1999 using a new "capabilities based" model to prepare itself for uncertain future military threats. The events of 9/11 and the declaration of the war on terror in 2001 again refocused the nation and the Army on the role of ground forces in the 21st century. This study provides insights into how the US Army sought to prepare for the future at the end of major conflicts, and suggests approaches which Army leaders may wish to keep in mind as they continue to adapt to evolving circumstances and realities.