I present new evidence of the effects of climate shocks on conflict. Focusing on political violence in Africa, I find that El Ni\~no Southern Oscillation (ENSO) shocks during the crop-growing season affect harvest-related conflict in croplands exposed to this climate phenomenon. Specifically, a 1{\deg}C warming of sea surface temperature in the tropical Pacific Ocean, a proxy for a moderate-strength El Ni\~no event, reduces political violence in exposed locations with crop agriculture, relative to other areas, by approximately three percent, during the early postharvest season. This effect attenuates toward zero as the crop year progresses. This effect can be five times as large after a strong El Ni\~no event, such as that of 1997 or 2015, in highly exposed croplands, such as parts of Southern Africa and the Sahel. Conversely, a La Ni\~na event, which is a counterpart of an El Ni\~no event, has the opposite effect and thus increases conflict in the exposed croplands during the early postharvest season. Because ENSO events can be predicted at least several months in advance, these findings can contribute to creating a platform for early warnings of changes in political violence in predominantly agrarian societies.