During the second half of the 15th century and most of the 16th century, Portugal took on a prominent position in the European scene as the precursor of the great oceanic voyages which since then regularly connected Europe to South America, the Indian Ocean, China and Japan. With the limited means of a small country, positioned at the western edge of the Iberian Peninsula, Portugal was able to build up an extraordinary maritime capability that lasted several decades against nearly all rivals. Naturally, this is due to its geographical position in the southwest extreme of Europe, facing the Atlantic Ocean, with no other choice but to turn to that ocean, which conditioned its destiny and became the reason for its political independence. This capability was built from the first moments of its existence as a nation and valued over time, with a set of other assets that, along with its geography, ended up being decisive. In the present essay I will attempt to explain this process, which went through several decisive stages, from independence in the 12th century, to the beginning of the maritime expansion with the exploration of the Atlantic and the discovery of the passage to India through the Cape of Good Hope. These are the stages I will attempt to highlight, explaining how they were lived and illustrating how they are related, even when they are separated by several decades.