Soybean plays a prominent role in the Brazilian economy. Its rapid expansion reached the Amazon in the 2000s, replacing pastures and indirectly influencing deforestation. We analyzed the importance of different natural, economic and political factors in the expansion of soybean and its links with deforestation, by studying the Santarém Plateau located in the central Amazon. The temporal links between the areas under soybean and forest and explanatory factors were studied using public census data, satellite time series and document analysis. Results showed that soybean expansion and deforestation were multifactorial. Initially, the government chose this area for its fertile soils and the favorable climate. The arrival of soybean in 1996 was anticipated through public research to find suitable cultivars, bank loans and facilitated access to land, despite irregularities in land tenure. The construction of the grain port by an international trade group in 2003 made exporting grain possible by considerably reducing transport costs and stimulated investments by farmers arriving from other Brazilian states. The subsequent expansion of soybean area mirrored the increase in its selling price. The social impacts were considerable: local communities of family farmers were swiftly and brutally expelled from their land to make way for large-scale grain cultivation. The environmental command-and-control implemented by the government and the soybean sector under the pressure of environmental NGOs limited the impacts of soybean expansion on deforestation between 2006 and 2014. But since 2015, the weakening of control has been accompanied by a resumption of deforestation.