The political and cultural entity that constitutes modern Spain did not exist in antiquity and came into being during the medieval period only gradually. The Iberian peninsula was populated far back into the Paleolithic period but those shadowy, indigenous peoples of Iberia had been overlaid in part and in turn by Phoenecian, by Carthaginian, by Greek, and by Celt. But of these intruders none had brought unity until Rome gradually had extended her control over all of its peoples, beginning with the Second Punic War (218–201 bc). During the next six hundred years Roman rule there became an imperial order and the fundamental structure of the Roman Empire was always the province. Still at the beginning of the Middle Ages Iberia was simply a collection of Roman provinces politically, the westernmost peninsula of the Roman world geographically, and a participant in a common culture which we style classical.

The geography had imposed its conditions upon the Romans as it had and would upon everyone who attempted to govern Iberia. By European standards, the Iberian peninsula is large, more than 581,000 square kilometers. That is more than four times the size of England and a little bigger than France. Iberia is also very dry, on the average, with an annual precipitation of less than 1,000 millimeters. The westerlies off the Atlantic see that the northern and western coasts are mostly well watered, but the mountains of Cantabria in the north and of Galicia and Portugal in the west drain the Atlantic winds of most of their moisture before they reach the interior.