Scholars of civil war and insurgency have long posited that insurgent organizations and their state enemies incur costs for the collateral damage they cause. We provide the first direct quantitative evidence that wartime informing is affected by civilian casualties. Using newly declassified data on tip flow to Coalition forces in Iraq we find that information flow goes down after government forces inadvertently kill civilians and it goes up when insurgents do so. These results have strong policy implications; confirm a relationship long posited in the theoretical literature on insurgency; and are consistent with a broad range of circumstantial evidence on the topic.