I feel that part of what
@Lalli was implying was that because of the different marriage customs of Europe it would be far more likely that the direct Merovingian bloodline would be more likely to eventually go extinct, although I'm not sure to what extent the Merovingians considered descent through auxiliary bloodlines acceptable.
In any case, would the Merovingians really rule over "essentially everything"? The Frankish kingdom didn't control Iberia, the British Isles, or other substantial portions of Christendom, much less all of Europe and the Mediterranean basin, and I doubt that the Merovingians would have been any more capable of conquering all of them than the Carolingians; less, perhaps, because of how the monarchs attended themselves to ritual concerns instead of state concerns. Sure, pushing vassals outwards works up to a point, but eventually you run into obstacles that require more centralized leadership and the vassals themselves start to fight with each other more than with external enemies.
Well, the Capetians ruled more or less four times longer than the Merovingians, so it would seem that their approach worked much better.
This sounds like a lot of Chinese philosophy, but the idea didn't really work there, either. Really, it only worked in Japan, for reasons that aren't totally clear to me.
It did work though in Europe, firstly for part of the Merovingian period and to some degree as part of Papal dogmatic philosophy. There are much discussions on this, in the comparison of the ideals of a sort of mystical bloodline of ceremonial ritual kings in the Merovingian sense to that of the Japanese system of feudalism and of the doctrines of legalism. regarding legalism, in reference, to what I understand as the role of the king as ensuring certain ritual and of enforcing more a strict adherence to laws and customs. I believe the axiom was to enforce fear, terror and duty upon the bureaucracy so that they would work without direct intervention of the monarch. It would seem that the Qin preeminent eunuch, Zhao Gao operated under this system, despite his terrible reputation in later Han historiography. In a sense, the Merovingian also were operating under this system, with the nobles taking a certain fear, not due to the possibility of execution, but of a sort of curse that may be imposed upon he who obstructed the royal bloodline and its connection to the ritual of kingship. Indeed, one may say this was the case, when you consider the fate of European monarchs, either crushed outright by Papal legal interdiction, destroyed in revolutions, controlled by the Papacy into a marriage custom that often saw power loss quickly, generally embattled by the Papacy or unable to realize the ideal of universalism that most of the monarchs in Europe espoused.
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Regarding ages and times of rule, it is different. The Capet model was good, surely, it required their submission to the Papacy, which buttressed their government and actively held the French monarchy up, assisting it against all of its neighbors and vassals. Indeed, the Capet advocated a Merovingian motif wherein they asserted 'rule by the bloodline' rather than through divine appointment or by effective power. Combining this with the Papal power, the most effective monarchy and political order in Europe after the Merovingians and excluding the Eastern Empire, we understand readily why the Capet did so well.
Also, it is not known how long the Merovingians ruled the Franks. If we went by their myths, they ruled the Franks for centuries prior. What is more likely, is that the Merovingians were the first elected leading household of the Franks during their formation around 300-360 CE. At least of the Salian Franks across the Rhine. Likely, these nobles had been ruling the people thus as part of its ruling caste for around 470 years perhaps. This is not something to snuff at, especially when you understand that Capet success derives from the same roots as the Merovingians, except less pronounced. The Capet kings did not attempt to reinvent the wheel as the varied Holy Roman Emperors did and create some sort of sacred Roman empire with Byzantine flair in the middle of Frankish Europe.
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Regarding Europe, when we speak of everything, I refer to the Frankish world of the High Middle Ages, which was France, the Holy Roman Empire and to a lesser degree, Northern Italy. Merovingian expansion was more dramatic and expanded in more difficult areas than the Carloginian successors. Setting aside Clovis I, the Merovingians were able to subjugate many different Germanic peoples of diverse religions and language and formulated these under a greater Merovingian aegis. Not to mention forming the bedrock of the entirety of Medieval Europe through its synthesis of Frankish custom with that of traditional Latin religion.
The Merovingian kingdom expanded in autonomous ways in primarily the east, where it managed to conquer the varied Saxon peoples, without leading to bloody rebellion and disunity as under Charles I. Furthermore, effective superiority was established over the Lombard kingdom without having to conquer them. Given appropriate time and development, the demographic depth and colonization process of the Frankish world at the time, would permit successful expansions in all directions, just as was occurring in the Middle Ages. Indeed, European expansions in the Middle Ages, much more spectacular than Charles I, were performed culturally and militarily from the idea of a coalition lord, i.e the Papacy. In atl, this coalition lord is the Merovingian king and possibly the royal court, made up of the Church as the bureaucracy and the nobles who have privileges at court. Merovingian kings would be in essence, an unmovable symbol of Frankish unity, unlike Charles I, who was a symbol of regicide and of poor centralization methods.
So, the Merovingian expansion does not require a single centralized state, it only needs the feudal conception of a centrality that hypothetically exists that can make the calls to war. Then it can work like the Arsacid, Sassanid or indeed the Papacy in empowering an expansionist policy. Figurehead monarchs are not necessarily weak, they are figures which can act as rallying points and this would be one of the main rituals of the Merovingian monarchs, namely, each day, embodying Frankish eternal unity and expansion in a palatial-plunder economy that by its nature requires constant growth. Now, in such a society, we may not see the development of a commercial economy the same as otl, but we at least have an interesting situation.
And it does not matter if the vassals fight each other. The reason that it is less likely, is that the nature of Frankish warfare, that is generally relying upon plunder, necessitated the need to find prey who possessed wealth sufficient. The Frankish nobles in the period in question before the commercial economy, often transferred their wealth into movable treasures which were then interred into shrines or other religious sites of veneration. As the taboo was, one could not attack their comrades in their manors and also they could not loot shrines or religious sites (as this was a bad omen, which incurred curses), the Franks found their loot beyond the Frankish realm. This custom was still at play in Europe in the Middle Ages, once the Frankish world was back on the offensive as the Arab chroniclers explain, they expanded in order to reintroduce a plunder economy, that sought to destroy opposing foes beyond their frontiers, redistributing the wealth into the interior and in time, colonizing the areas for their civilizational complex. Indeed, this was the situation of the Merovingians, as they were the ones who introduced this as this model was certainly not Roman.
In the Middle Ages, at the height of weak Capet and Imperial monarchies, the Papacy describes infighting amongst the Franks less as wars, rather than vendetta squabbles. Urban II and his ilk likely understood that once possessing an effective figurehead (the Papacy) and a goal (renewed expansionism), these vassals could be formed into very powerful armies that in coalition could turn the tide on behalf of the Eastern Empire, just as they were doing in Iberia, along the frontiers with the Slavic peoples and against the Magyar. So, I do believe that there will be internal bloodletting especially later, but these will not be to overthrow the Merovingians, but rather wars to gain access closer to the court and access the supposed power imbued by Merovingian rituals.
One idea that later Medieval writers failed to understand is that the Merovingian 'do-nothing' but rituals mentality was understood as something that was magical, that produced certain exchanges in the real world that actualized power. It was a soft power that imposed itself across the Frankish realms. One example is how the Merovingian kings interacted as directly Intune with every caste in society and practicing a 'constant court' which gave legitimacy to all the nobility. Or the travelling in chariot for total ceremonial purposes, alongside pre-planned ceremonial hunts, all with the supposed benefit of maintaining the life of the state through ritual. However, this notion was rejected by the 'effective power' doctrine of Charles I. Who felt that the king must focus actively upon ruling, instead of taking the role as a benevolent figurehead, whose role is ritual imposition of magical order through repetition of ceremony. So vassals would indeed eventually fight over gaining better access to this as the more vassals the realm acquires, the more impossible it will be for all to hold court positions and hence fighting will commence over seats at court and thus abilities to receive the powers imbued by the Merovingian rituals.