This is the brief story of a decision process and the role of Strategic Environmental Assessment in government political decision-making. Following a prolonged, and agitated, decision process, initiated in the 1960s, the Government of Portugal in 2005 took the final decision to build the new international airport of Lisbon at the controversial location of Ota, 40km north of Lisbon. The detailed project design and EIA were started. However this decision would change in 2007 due to the challenge raised by a private sponsored study that identified an alternative location for the airport at Campo de Tiro de Alcochete (CTA). This new site, which had never been considered as an option before, appeared to avoid many of the problems that caused public controversy at the Ota site. The Government, pressured by this challenge, promoted a strategic comparative assessment between the two sites. The result of this study was the choice of CTA as the preferred location. This paper discusses this radical change in the decision from a socio-political perspective. It will highlight the relevance of Strategic Environmental Assessment (SEA), and the strategic and constructive approach it enables in mega-project decision-making.