The Reservation Point — RP — captures the value of achieving a set of war aims and Expected Total Costs — ETC — captures a war’s anticipated costs. Both vary across persons and conflicts and together determine the relationship between military casualties and opinion. Not achieving the aims makes costs unacceptable, endogenizing winning and losing. Variations in casualty expectations over time and space lead the political consequences of war to differ temporally and geographically. The role of casualties grows over the duration of the conflict. Marginal casualties will have a proportionally greater effect on the population’s estimates of total expected casualties (ETC) at the start and cumulative casualties over the duration of a conflict. Individuals’ values of wars and their aims fluctuate, which means that opposition will be triggered at different levels of expected and observed costs. The distribution of beliefs about the value of a conflict’s war aims and the distribution of ETC — both of which may change within a specific conflict, and clearly vary across them — are the primary (although not unique) factors that shape elite and mass approval.

People with extreme views appear to have fixed positions, but actually reflect calculations unlikely to be swayed by wartime information. The Evaluative Public, whose valuations are not in the tails, use wartime information to form their conflict approval. Evaluators are sensitive to changes in beliefs about observed casualty levels and expectations of likely future casualties. The Evaluative Public represents those who change positions in war, from support to opposition, depending on their ETC and RP. Using historical data and innovative experiments, we demonstrate that people’s personal experience with war drives their estimates of total casualties. Holding goals constant, higher costs yield higher opposition while lower costs increase support. Holding costs constant and allowing goals to change leads to opinion change. Geographically and temporally proximate casualties strongly influence estimates of a war’s total costs. Thus, we see that casualty patterns affect people’s estimates of a war’s costs and that these costs, when contextualized with the value of a conflict and expected costs, shape wartime support, even in the face of strong individual-level characteristics.