Early European accounts of East Africa often paid greater attention to the natural than to the human environment, often mistakenly assuming that large expanses of territory teeming with wildlife were vacant of people. During the colonial period, attention to environmental issues grew, especially concerns about soil erosion, overgrazing, and deforestation, and it empowered local officials to intervene in livelihoods and natural resource use, often with draconian measures. They subscribed to global narratives about Africans and their role in environmental degradation. Many of these narratives and ‘scientific’ ideas have endured into the post-colonial era and strongly impact land use and environmental policies in pastoralist areas of East Africa. This paper addresses one of these powerful tropes---desertification—that spurred a complex set of environmental rehabilitation experiments in East Africa, with the introduction of the invasive plant species prosopis juniflora being among the most pernicious. Although these forced environmental interventions were originally designed to combat perceived environmental degradation (i.e., the “advancing desert”), they are now layered with arguments about climate change and the “greening” of damaged landscapes. By drawing on the author’s long-term research in northern Kenya, the paper uses the example of a forestry project based on prosopis in Baringo County, Kenya, to explore wider issues of significance to the anthropology of pastoralism, including the hegemony of Western science, local enviornmental knowledge, human rights, and state-mandated development interventions. The paper employs ethnographic, archival, and photographic/visual materials to demonstrate the broad regional and national political ramifications of what was initially conceived to be a non-political technical intervention. The paper also suggests that longitudinal studies which span decades rather than years can contribute much to anthropological theory and method.