For more than 2 decades, the Strategic Studies Institute (SSI) has hosted an annual strategy conference. Each conference was designed to convene some of the world's top experts on a major strategic issue, and to use cutting edge scholarship and analysis to help the U.S. Army and Department of Defense (DoD) leadership understand the issue. The April 2010 Strategy Conference was entitled "Defining War for the 21st Century." The conference included a keynote address by Professor Martin van Creveld, a banquet presentation by Major General (Retired) Robert Scales, and panels on the historical context; the instigation of war; the end of wars; the participants in war; the rule sets governing war; and the policy, strategy, and organizational implications of defining war. The conference speakers, which included well known scholars, former policymakers, and former senior military leaders, agreed on some points, but often had very different perspectives. Most importantly, they identified the most pressing questions that the American and international defense communities are grappling with as they refine their definition of war. In the report which follows, Steven Metz and Philip Cuccia of SSI have summarized the presentations and debates at the conference and placed them in their wider intellectual and strategic context. SSI is pleased to offer this report in fulfillment of its mission to assist U.S. Army and DoD senior leaders and strategic thinkers in understanding the key issues of the day.