This article seeks to provide a panorama of the reasons leading to the creation of foreign bases in modern Portugal. By foreign bases is meant the prolonged presence (of at least one year) of foreign military units that have been duly authorised by the legitimate authorities on Portuguese territory. The presence of military missions, even when large-scale and prolonged, or of training or instruction missions will not be considered here. This study covers the period from the establishment of the multi-polar international system at the end of the 19th century until the transition to democracy in 1974, which also corresponded with the final phase of the Cold War. Only European Portugal, which includes Madeira and the Azores, will be examined: the colonies will not be considered. The way of understanding the establishment of foreign military bases in Portugal is to analyse the national and international strategies that resulted in their creation. The military plans that these strategies inspire are mentioned; however, lack of space here means they cannot be explored in any exhaustive or developed manner: the bases are classed according to the strategies that led to their creation.