We exploit the seasonality of agricultural labor markets to estimate the effect of changes in the returns to working versus fighting on the intensity of conflict. Based on a dynamic model of labor supply we show theoretically that exogenous, anticipated, and transitory changes in labor demand due to harvest are better able to capture the intra-temporal substitution between labor and conflict than a number of other instruments in the literature. This is because seasonal shocks hold constant factors commonly omitted in empirical studies - such as the present value of future victory or the marginal utility of consumption. Our identification strategy exploits exogenous sub-national variation in the timing and intensity of harvest driven by local climatic conditions. Using data from four different conflict settings - Afghanistan, Peru, Iraq, and Pakistan - our results show that the onset of harvest usually leads to a statistically significant reduction in the number of monthly insurgent attacks. These results document the ubiquity of opportunity costs mechanisms across different conflict settings.