With Clovis' conversion to Catholicism in 498

At the end of the Antiquity period, ancient Gaul was divided into several Germanic kingdoms (Early Francia (North), Alamannia (North-East), Burgundia (East), Septimania (South), Visigothic Aquitania (South East)) and a remaining Gallo-Roman territory, known as the Kingdom of Syagrius (West). Simultaneously, Celtic Britons, fleeing the Anglo-Saxon invasion of Britannia, settled the western part of Armorica (far West of Gaul). As a result, the Armorican peninsula was renamed Brittany, Celtic culture was revived and independent petty kingdoms arose in this region.
The pagan Franks, from whom the ancient name of "Francie" was derived, originally settled the northern part of Gaul, but under Clovis I conquered most of the other kingdoms in northern and central Gaul. In 498, Clovis I was the first Germanic conqueror after the fall of the Roman Empire to convert to Catholic Christianity, rather than Arianism; thus France was given the title "Eldest daughter of the Church" (French: La fille aînée de l’Église) by the papacy,[33] and the French kings would be called "the Most Christian Kings of France" (Rex Christianissimus).


With Clovis' conversion to Catholicism in 498, the Frankish monarchy, elective and secular until then, became hereditary and of divine right.
The Franks embraced the Christian Gallo-Roman heritage and ancient Gaul was eventually renamed Francia ("Land of the Franks"). The Germanic Franks adopted Romanic languages, except in northern Gaul where Roman settlements were less dense and where Germanic languages emerged. Clovis made Paris his capital and established the Merovingian dynasty, but his kingdom would not survive his death. The Franks treated land purely as a private possession and divided it among their heirs, so four kingdoms emerged from Clovis's: Paris, Orléans, Soissons, and Rheims.
The last Merovingian kings, sometimes referred as Rois fainéants ("lazy kings"), effectively lost power to their mayors of the palace. One mayor of the palace, Charles Martel, defeated a Muslim invasion force from Hispania at the Battle of Tours (732) and earned respect and power within the Frankish kingdoms. His son, Pepin the Short, eventually seized the crown of Francia from the weakened Merovingians and founded the Carolingian dynasty. Pepin's son, Charlemagne, reunited the Frankish kingdoms and built a vast empire across Western and Central Europe.
Proclaimed Holy Roman Emperor by Pope Leo III and thus establishing in earnest the French government's longtime historical association with the Roman Catholic Church,[34] Charlemagne tried to revive the Western Roman Empire and its cultural grandeur, from his Palace of Aachen. The efficient administration of this immense empire was ensured by high-level civil servants, carrying the, then non-hereditary, titles of counts (in charge of a County), marquis (in charge of a March), dukes (military commanders), etc.


French territorial evolution from 985 to 1947.
Charlemagne's son, Louis I (emperor 814–840), kept the empire united; however, this Carolingian Empire would not survive his death. In 843, under the Treaty of Verdun, the empire was divided between Louis' three sons, with East Francia going to Louis the German, Middle Francia to Lothair I, and West Francia to Charles the Bald. Western Francia approximated the area occupied by, and was the precursor, to modern France.[35]
During the course of the 9th and 10th centuries, continually threatened by Viking invasions, France became a very decentralised state: the nobility's titles and lands became hereditary, and the authority of the king became more religious than secular and thus was less effective and constantly challenged by powerful noblemen. Thus was established feudalism in France. Over time, some of the king's vassals would grow so powerful that they often posed a threat to the king. For example, after the Battle of Hastings in 1066, the Duke of Normandy added "King of England" to his titles, becoming both the vassal to (as Duke of Normandy) and the equal of (as king of England) the king of France.