Much has been written about the national-security aspects of a potential conflict with Iraq, but there are no studies of the cost. A review of several past wars indicates that nations historically have consistently underestimated the cost of military conflicts. This study reviews the potential costs of a conflict including the postwar expenses that might be required for occupation, humanitarian assistance, reconstruction, nation-building, along with the implications for oil markets and macroeconomic activity. It considers two potential scenarios that span the potential outcomes, ranging from a short and relatively conflict-free case to protracted conflict with difficult and expensive postwar reconstruction and occupation. The estimates of the cost to the United States over the decade following hostilities range from $100 billion to $1.9 trillion.