The RAND Arroyo Center conducted a study for the U.S. Army titled Defense Planning in a Decade of Conflict. The purpose of the project was to perform a comparative historical review of the four Quadrennial Defense Reviews (QDRs) conducted since the first QDR in 1997 - including QDR reports in September 2001, February 2006, February 2010, and March 2014 - to identify larger trends, as well as implications and recommendations for the Army to shape the conduct of and thereby improve future reviews. The main product of that project was a report - Defense Planning in a Time of Conflict: A Comparative Analysis of the 2001-2014 Quadrennial Defense Reviews, and Implications for the Army - that documented the results of our analysis to Army and Department of Defense senior leaders and planners well enough in advance that measures can be taken now to improve the organization, processes, and analytics associated with the next Defense Strategy Review. This report provides a stand-alone Executive Summary that captures the key findings and recommendations of the larger report. Both the main report and the Executive Summary may be of interest to defense planners in the U.S. Army, Office of the Secretary of Defense, and Joint Staff, as well as students of defense planning in the scholarly community.