Between the third and the fifth centuries, some were allies of Rome and some harried the Empire. Around 250, one group of Franks, taking advantage of a weakened Roman Empire, penetrated as far as Tarragona in present-day Spain, plaguing this region for about a decade before Roman forces subdued them and expelled them from Roman territory. About forty years later, Franks controlled the region of the Scheldt River and were raiding the Channel, disrupting transportation to Britain. Roman forces pacified the region, but did not expel the Franks, who continued to be feared as pirates along the shores at least till the time of Julian the Apostate (358), when Salian Franks were granted to settle as foederati in Toxandria. By the end of the fifth century, the Salian Franks extended their footprint on Roman soil to a territory including the Netherlands south of the Rhine, Belgium, and Northern France in which they received other peoples, mainly other Franks.

In the 5th century the Merovingian dynasty arose. Of these, the first to declare himself "King of the Franks" was Clovis I in 509. He had taken the Kingdom of Soissons from the Roman general Syagrius and expelled the Visigoths from southern Gaul at the Battle of Vouillé, thus establishing Frankish hegemony over most of Gaul (excluding Burgundy, Provence, and Brittany). Clovis divided his realm between his four sons. They united to defeat Burgundy in 534, but internecine feuding came to the fore during the reigns of the brothers Sigebert I and Chilperic I and their sons and grandsons, largely fueled by the rivalry of the queens Fredegunda and Brunhilda. This period saw the emergence of three distinct regna (realms or subkingdoms): Austrasia, Neustria, and Burgundy. Each region developed in its own way and often sought to exert influence over the others. The Arnulfing clan of Austrasia gained strength at the expense of the others so that the center of political power shifted east from Paris and Tours to the Rhineland.

In 613 Chlothar II, son of Chilperic reunited the Frankish kingdom and granted the Edict of Paris to the nobles in an effort to cut down on corruption. However, after the militarily successful reign of his son and successor Dagobert I, royal authority rapidly declined under a series of kings traditionally known as rois fainéants ("do-nothing kings"). By 687, after the Battle of Tertry, the chronicler could say that the mayor of the palace, formerly the king's chief household official, "reigned." Finally, in 751, with the approval of the papacy and the nobility, the mayor Pepin the Short, son of the mayor Charles Martel, deposed the last Merovingian king, Childeric III, and had himself crowned Pepin III, inaugurating a new dynasty, the Carolingians.

Under the Carolingians, most of what is now western and central Europe was united and Roman Catholicism was spread throughout the area. Despite almost constant internecine warfare, the period is referred to as the "Carolingian Renaissance." After 800, when the Pope crowned Charlemagne Holy Roman Emperor, it was seen as the continuation of the Roman Empire in the West. Each part of the Carolingian Empire developed differently; Frankish government and culture depended very much upon individual rulers and their aims. Those aims shifted as easily as the changing political alliances within the Frankish leading families. However, those families, the Carolingians included, all shared the same basic beliefs and ideas of government. These ideas and beliefs had their roots in a background that drew from both Roman and Germanic tradition, a tradition that began before the Carolingian ascent and continued to some extent even after the deaths of Louis the Pious and his sons.