I’m currently reading Charlemagne: Father of a Continent (2000, 2004 translation) by Italian historian Alessandro Barbero. I can strongly recommend this book, which provides military history as well as insight on the governance of King Charles (Charlemagne)’s empire. It does go into some detail about the wars in Spain vs those against the pagans. All information here is taken from that book.

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TLDR: Charles ruled directly and was in closer proximity to Saxony. He carried out totaly war in order to conquest Saxony and other territories in central and eastern europe. He made advances in Spain, from Basque country to Barcelona, which forms the basis of modern Catalonia. Spain and Lombardi were both officially governed by his sons, as Kings of Aquitaine and Lombard. Further advances in Spain were ineffective because the Arabs kept to heavily fortified cities, and Charles apparently never took much of an interest in going over the Pyrenees and fighting there himself, since he was busy utterly crushing the Saxons and bringing him under his direct control. He also was engaged in an effort to destroy Pagan culture and enforce Christian values, while he had good relations with the Muslim caliphate, although that doesn’t seem particularly significant to why he didn’t go deeper into Spain.

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First, what I think is notable is the structure of the Frankish empire. Charles never had a centralized capitol, but travelled throughout Frankish lands with his retinue. He generally was at war in the summer, had festivals and holidays in the winter, and held court (resolving legal disputes and other matters) wherever he happened to end up in the spring.

Frankish territories were governed by a mixture of his lieutenants that all reported back to him. Counts and marqueses (whose territory was called a ‘march’) were technically the ‘governors,’ but were dispatched in large numbers, and were required to report to the bishops. Charles could also specially send out emissaries (vassals and missuses) who reported back directly to Charles. The bishops, counts, and vassals all had competing powers, and all had intimate relations to Charles, giving him the ultimate authority over Frankish lands. This obviously depended highly on Charles’ skill as a leader and his personal relationship with these lieutenants, which might (and this is pure speculation on my part) explain why his empire fell apart after his death.

It is important to note that the situation was a little bit different in Lombard (Italy) and in Aquitane (which currently occupies the borderland between Franch and Spain). Charles’s son Louis was made King of Aquitane, while Charles conquered Lombard and ruled it through a combination of the Lombardi rulers who preceded him, after 774, and then installed some Franks to oversee the Lombardi apparatus following a revolt in 776. His son Pepin was eventually named King of the Lombards.

Charles also had an ally in Pope Leo III. Leo was not of noble birth, and owed his papacy (and likely his life) to Charles, who saved him after more noble families loyal to Leo’ predecessor attempted to replace Leo. (Leo fled from Rome on account of this conspiracy, and it was rumored that his eyes and tongue were cut out. Leo told Charles they miraculously grew back.) Leo’s debt to Charles led to Charles’ coronation as Emperor, making him almost the de facto church leader.

Charlemagne is said largely to have a defensive attitude towards Muslim Spain, choosing to defend Aquitane rather than carry out excursions in Spain proper. As in Lombard, Charles did not rule so directly as he did in Frankish lands. Instead, his son Louis was installed as King in 781, granting some recognition to Aquitane autonomy (and winning their loyalty in the process).

In 778, however, Charles DID attempt an incursion into Spain, on account of some disunity seen in Spanish lands. Christians in the north of Spain, including the Basques, saw Charles as their protector, and so requested help on multiple occassions. Charles was personally able to lead an army over the Pyrannes and into Basque country in 778, while in 800 Louis, with support from Charles, was able to lay seige to Barcelona and eventually capture it.

So Charlemagne did, in fact, capture some territory in Northern Spain. Barbero writes: “The area taken from the Muslims was incorporated into the empire and organized militarily as a foritifed frontier provenince, or march, to use the terminology of the imperial administration. This Spanish march . . . gave rise to its own particular features that are today recognizable in Catalonia, such as the closeness of its language and customs to southern France. It was traditionally considered more European than the rest of Spain.” (60)

Barbero does not seem to get much into why the Frankish empire did not expand further into the peninsula, except to write: “even though they were fighting in their own territory against an enemy that had considerable logistical problems, Arabs always avoided pitched battles and relied for their defense exclusively on their city walls. This reliance was by no means misplaced, because in such a highly urbanized region, the Franks found making progress an extremely slow business, particularly given the crudeness of their siege equipment.” (60-61).

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So after surveying the situation in Lombardi and Aquitane / Spain, there are some key distinctions that can be drawn to Saxony. The largest, as I see it, being: (1) their were not mountains separating Frankish lands from Saxony, as there with the Lombards and Spanish; (2) Charles ruled directly over Saxony (through his system of Marches and other delegates), rather than assigning one of his sons to be King of the Saxons; and (3) Charles spent a great amount of time on the field directing his armies in Saxony, where the other battles were fought through a greater degree of delegation.

Again, while Charles never had a ‘capitol’ per se, the closest thing to it was Aachen, which he built up as sort of a permanent home, but never made it a See. Aachen happens to be in modern-day Germany, and was on the edge of Frankish territory closer to Saxony. Which is further evidence of Charles’ direct proximity to that field of battle.

The Saxon war was a guerilla war, lacking the elements of fortified cities in Spain, nor existing urban governance structure that could be taken over by the Franks, as in Lombard. In response, Charles carried out a sustained scorched-earth campaign against the Saxons. Barbero describes Charles as wanting “to act like a true king of Israel” on this front . . . After defeating the Moabites, David, with whom Charles liked to compare himself, had the prisoners stretch out on the ground, and two out of three were killed.” (47) It is in this model that Charles decapitated 4,500 Saxons in a single down at Verden in 782. For the first time, he wintered in enemy territory that year “and systematically laid the country waste to starve the rebels” and “imposed the death penalty on anyone who offended the Christian religion and its clergy, and in reality it constituted a program for the forced conversion of the Saxons.” This law was so harsh that anyone who failed to observe the fasting on Friday would be put to death. Charles himself would later lead his armies through Bavaria, and then onwards to destroy the Avar Khanagate, a Central European power that Charles also personally destoryed.

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In sum, it seems that Spain (and Italy / Lombard) were more urbanized and more developed politically. This allowed Charles to conquer and maintain control over those territories by toppling rulers and then installing his sons to rule over the existing apparatuses. Charles was able to win over these areas culturally by granting recognition to the independence of the people of Aquitane, by appearing as a true Christian leader to the Basque in Northern Spain, and by having the firm loyalty of the Pope and the local Kings in Lombard (where it also helped that Charles was quickly able to put down rebellions were they appeared). These territories were also much further geographically from Charles, not to mention separated by mountains.

Charles apparently had no such means for control in Saxony (and Bavaria, and over the Avars). More significantly, Charles led the campaigns against these territories personally - they were close to his traditional ‘seat’ (as little as he used it), and were ruled more directly by him rather than through one of his sons named as King. In order to conquest these lands, Charles carried out scorched earth campaigns to destroy the pagans and implement Christianity (of which he was the clear head at the time, higher than the Pope in actuality, even if not theologically).

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FINALLY, religion would not have been a major factor, at least as we see Christians versus Muslims today. Charles waged what could easily be called a holy war against the Saxon pagans, but maintained good relations with Byzantine Empress Irene of Athens (who blinded her own son to take power), except when Charles broke his promise to send one of his daughter’s to marry Irene’s son (Charles was apparently very fond of his daughters).

He also maintained good relations with Harun Al-Rashid, the Caliph who sat in Baghdad. The Caliph sent Charles an elephant named Abdul-Abbas, which Charles took with him everywhere (until the death of the creature), and which was quite a thing of interest wherever it went. Apparently Charles sent some wolves and other tributes to the Caliph, but those weren’t taken with as much interest in Baghdad.