Einhard (also Eginhard), a German historian and advisor to Charlemagne (Thomas 2), left us Vita Caroli, a record of the king's life which includes a description of his conquests and failures in Spain. While the record cannot be considered historically objective as the West now understands it, Vita Caroli is the only extant contemporary text describing Charlemagne's life and is thus of necessity accepted as fact, with reservations. Several sources, including Italo Calvino in his version of the Orlando Furioso and Glyn S. Burgess in his translation of The Song of Roland, refer to Einhard's information as their primary historical source. All the information, therefore, that is presented below as historical is taken from these references to Einhard and is presented with the usual reservations as to possible inaccuracy or subjectivity on the part of a historian who wrote a millennium ago.
In 778 Charlemagne arrived in Spain at the request of the Muslim governor of Barcelona, who needed help in defeating his rival in Saragossa. (The reward for Charles, aside from the personal satisfaction of having helped someone in need, would be control over several cities in Spain.) While crossing the Pyrenees on their way back to France in August of that year, the Franks were ambushed by the Basques, who resided in the area. In the course of the battle, in which all of the army's rearguard was destroyed, died Lord Hruodlandus of Brittany.
We do not have much information on the historical Roland, except that he was in the service of King Charlemagne of France, served as captain on the Breton border and died in an ambush by the Basques in 778 A.D.