What determines a democratic public's willingness to tolerate the human and material costs of sustaining ongoing military operations to victory? Athough much literature has addressed the factors that affect public attitudes toward the use of military force, few studies adopt either a theoretical perspective or a research method explicitly designed to answer this question. In particular, existing research tends to focus on the costs of war fighting, while ignoring both the tangible and intangible costs of withdrawing from a foreign military engagement. I argue that many of the factors that the public uses to estimate the cost of prosecuting a war—troop strength requirements, whether or not troops are engaged in ground combat and, most importantly, casualties—are also measures of the extent of a state's commitment to achieving its war aims. If the public treats the cost of the state's military commitment simply as an expense , support for sustaining an operation should decrease as the cost of commitment increases. If, however, citizens have a tendency to see military commitments as investments that put the country's reputation on the line or can only be redeemed if the state is victorious in the war, an increase in commitment could actually strengthen the public's determination to sustain the fight. Employing a cross-sectional time-series design with data from 12 U.S. and British military interventions, I explore whether the costs of continuing to prosecute a war or the costs of withdrawing have a greater effect on public willingness to sustain ongoing military operations. The results suggest that public concern about the costs of withdrawing from a conflict can be a more important determinant of willingness to persevere than sensitivity to the costs of war fighting. As a result, there is a considerable disconnect between what the public claims it would support in hypothetical scenarios and the types of military operations the public actually shows a willingness to sustain once they are underway.