How did Charlemagne establish the Spanish March?

Charlemagne established the Spanish March as a militarized buffer zone between the Pyrenees and the Ebro River following an initial, unsuccessful military venture into Spain in 778.12

Although his 778 campaign ended in a disastrous defeat by Basque forces at Roncesvalles, Charlemagne remained committed to securing his southern frontier. To better manage this region, he created a subkingdom of Aquitaine in 781, appointing his son Louis as king. From this base, Frankish forces launched a series of campaigns that eventually succeeded in establishing control over the territory, creating a stable, militarized buffer against Muslim forces in Spain. This effort represented an early, significant step in the eventual Reconquista of the Iberian Peninsula.231