The kings of France did eventually acquire all the land in what we think of as “France” through inheritance and confiscation, but it was a bit more complicated than that, and they also needed to develop the legal and administrative apparatus to give them the power to hold onto all that land.

Basically way back in the 9th century, it was all part of Charlemagne’s empire, along with Italy and Germany and bits of Spain as well as bits of Eastern Europe. After Charlemagne’s death it pretty much immediately collapsed. In 843 his descendants divided it up into “West Francia”, “Middle Francia”, and “East Francia”. Middle and East Francia essentially became the Holy Roman Empire and West Francia became France. That’s an extreme simplification but that’s basically the situation in the 9th and 10th centuries.

West Francia also more or less immediately collapsed. There were rebellious Basques and Bretons, and from outside it was attacked by Normans and Muslims. The local nobles realized they couldn’t count on the support of the king, and if they couldn’t depend on the king, was the king really in charge? So the king held his own personal territory around Paris (the Île-de-France) but if he was unwilling or unable to assert his authority outside of his personal holdings, then everyone else was certainly willing to rule independently. The Duke of Burgundy, the Count of Champagne, the Count of Flanders, the Duke of Brittany, the Duke of Aquitaine - they were all theoretically vassals of the king, but in practise were completely separate. But the breakdown of authority didn’t stop there - especially in Aquitaine, local counts and lords fended for themselves. The land was often governed by “castellans”, which means there was no greater authority than whoever ruled the local castle. Also in the 10th century, the northern part of the king’s territory, basically between Paris and the sea, was granted to Norman invaders, so they had their own Duke of Normandy.

This started to change with the installation of the Capetian dynasty in 987 (the Capetian era from 987-1328 is basically the “high Middle Ages” for French history), but it still took quite a long time because most of western France actually came to be ruled by the king of England first. The duke of Normandy, William the Conqueror invaded England in 1066 so Normandy and England were ruled by the same person. Then the count of Anjou became king Henry II of England. Henry II married Eleanor, who inherited the Duchy of Aquitaine, and they passed these territories on their children, including the kings Richard and John. Up until 1204, more of “France” was ruled by the English king than the French king.

In 1204, Philip II of France took some of these territories by force, especially Normandy and Anjou. Simple conquest was one way for the king of France to expand his territory. So now who was the duke of Normandy and the count of Anjou? Well, no one. They were incorporated into the personal domain of Philip II. Philip was the duke and the count as well as king. If the English could do it that way, so could he.

Philip’s son Louis VIII and grandson Louis IX also conquered parts of Aquitaine. The county of Poitou was conquered from the English in 1223. By 1229, the Albigensian Crusade defeated religious heretics in the County of Toulouse. Toulouse was technically part of the Duchy of Aquitaine but had long been independent. This “crusade” was (probably correctly) considered a blatant power grab by the king. Toulouse was absorbed into the royal domain as well.

The next step was to dissociate all these territories from the personal property of the king, but still keep them in the possession of the royal family by granting them to the king’s brothers and sons, rather than letting the native dynasties of these territories continue to rule on their own. Just like the English had done! So for example, the sons of Louis VIII were Louis IX, the king; Robert, Count of Artois; Alphonse, Count of Poitou, and Charles, Count of Anjou. (Alphonse also married Joan, the heiress of the county of Toulouse, which ensured that Toulouse stayed connected to the royal domain, and when they both died in 1271, it became the king’s territory.) Territories that were handed out this way are called “apanages”.

Of course, handing over big important territories doesn’t necessarily mean they will stay part of the royal family’s territory forever. Obviously that had already happened with Charlemagne’s empire. It had also happened in the 11th century when the king of France Robert II gave the kingdom of France to one son, Henry I, and the Duchy of Burgundy to the other (also named Robert), as an apanage. But then for about 300 years Burgundy was essentially totally independent. The line of dukes died out in 1361 and it passed into the royal domain of France, but King John II gave it away again, to his son Philip, and the same process repeated all over again. Burgundy was independent, and in fact a major combatant in the Hundred Years’ War between France and England, before it was reabsorbed into France again in the 15th century.

I should also mention another marriage that brought a previously-independent duchy under French control. Anne of Brittany married two different French kings in the 16th century, and Brittany was attached to France after that, through her children with Louis XII.

Anyway, the reason this could happen, starting in the 13th century, was a revolution in administration, which was also borrowed from England. England was more compact and unified and therefore easier to govern and tax, and the English kings who were also dukes of Normandy and Aquitaine and other places in western France introduced the English system there too. Eventually they were adopted by the French as well, so they could govern their territories in the same centralized way as England. The way to do this was to replace the counts of Anjou or Toulouse or wherever else with a representative appointed by the king, who could have various names, a provost or a seneschal or something else. Then the territory was ruled according to the king’s law and taxes were collected and armies raised for the king, not for a local lord. It wasn’t perfect though since the legal system remained pretty chaotic. Up until the 18th century there actually wasn’t really one single legal system in France, there were hundreds of different systems, inherited from an older period when the different territories were independent.

So, territories could be brought under the king’s control by conquering them, or by marrying into their dynasties and absorbing them afterwards, but this went through several cycles and the “apanages” were sometimes lost and had to be re-absorbed. The real reason this could happen at all is that the French government was reformed, so it would have the authority, and more importantly the power, to impose royal governors and collect taxes, etc.

Sources:

Charles T. Wood, The French Apanages and the Capetian Monarchy, 1224–1328 (Harvard University Press, 1966)

A.W. Lewis, "The Capetian apanages and the nature of the French kingdom", in Journal of Medieval History 2 (1976), pg. 119–34

Elizabeth M. Hallam, Capetian France, 987-1328 (Longman, 1980, repr. 1990)

Jean Dunbabin, France in the Making, 843-1180 (Oxford University Press, 1985)