This paper examines how a boom in industrial cassava served as a 'gateway' to intensify capitalist relations in Cambodia's north eastern borderland. Situated on Cambodia's border with Vietnam, Mondulkiri province has experienced a rapid increase in cassava production and trade since 2006, with transformative consequences for the region's forests and farmers. Using field data from 2012 to 2014, we explore how the boom ignited and intensified over time, through a conjuncture of conditions. Alongside strong market demand for cassava, these included resource abundance (soil fertility, timber, land, labour), connectivity to markets and cross border networks, and facilitative governance conditions. Over time, the boom strengthened capitalist relations, particularly through farmer debt and the revalorisation and accumulation of land. However, unlike booms of tree crops elsewhere, we argue that it is the very impermanence of cassava that is formative here, because the crop's short-term nature and low overheads facilitate practices like land laundering and land mortgaging. Like the 'gateway drugs' that were believed to place users on a path to addiction and risk, this paper shows that gateway crops such as cassava may similarly place farmers on a trajectory of more intense competition and reduced choice in their engagements with capitalist modes of production.