A new threat now confronts the Amazon in the form of a massive infrastructure program, the Initiative for the Integration of the Regional Infrastructure of South America, or IIRSA. This article presents results of a projection analysis showing that IIRSA could push the Amazonian forest past a “tipping point,” replacing it with tropical savanna. Such an event would degrade biodiversity, reduce carbon storage, and harm continental agriculture, dependent on moisture transport from forest-based rainfall recycling.