The debate over counterinsurgency (COIN), seemingly dormant since the end of the Vietnam War, has been rekindled by the conflicts in Iraq and Afghanistan. Since the 2006 publication of the U.S. Army/ Marine Corps Field Manual (FM) 3-24, 'Counterinsurgency', practitioners and scholars have argued over the efficacy of COIN. Supporters insist that the new approach outlined in the manual led to the creation of a strategy that defeated the Iraqi insurgents between 2006-2009. Critics argue that the surge of 30,000 additional troops, robust conventional operations, and the end of the Shia uprising-not a new COIN strategy-caused violence in Iraq to decline dramatically. They point to the failure of the campaign in Afghanistan as further evidence that COIN does not work. In an era of declining Pentagon budgets, this debate has significant implications for U.S. land forces. This monograph considers the place of COIN in U.S. Army doctrine, training, and resource allocation. It begins with a brief overview of the U.S. military's historical experience combating insurgency before considering the recent campaigns in Iraq and Afghanistan. The monograph then examines in detail the contemporary, scholarly, and professional debate over the efficacy of COIN and its place in U.S. defense planning. Recognizing that consideration of this important issue must be grounded in an examination of the contemporary security environment, the monograph reviews official threat assessments. It then considers the current U.S. military capacity for addressing identified threats. That capacity includes force structure, doctrine, and learning institutions.