Most of the world’s coca—the source of cocaine—is grown in the Amazonian forests of Colombia, Peru, and Bolivia. As cultivation continues despite eradication, a shift to giving farmers more incentives to abandon coca is currently proposed. Assuming coca cultivation is an important cause of migration and deforestation, new alternative development projects also aim to conserve forests. We show coca cultivation strongly increases near never-completed 1960s–1970s state-sponsored projects to settle the Amazon. Improved roads and colonization projects opened the western Amazon frontier to migration, generating deforestation and, once support dwindled, setting the stage for coca cultivation. New studies also show coca cultivation generates negligible direct or indirect forest loss and fails to explain migration, whereas expanding legal agriculture, roads, displacement, and eradication increase deforestation. These findings highlight the urgent need to both commit development investment for the long term and set explicit conservation goals in western Amazonia.