Gauls
In 58 BC Julius Caesar launched the Gallic Wars and had conquered the whole of Gaul by 51 BC. He noted that the Gauls (Celtae) were one of the three primary peoples in the area, along with the Aquitanians and the Belgae. Caesar's motivation for the invasion seems to have been his need for gold to pay off his debts and for a successful military expedition to boost his political career. The people of Gaul could provide him with both. So much gold was looted from Gaul that after the war the price of gold fell by as much as 20%. While they were militarily just as brave as the Romans, the internal division between the Gallic tribes guaranteed an easy victory for Caesar, and Vercingetorix's attempt to unite the Gauls against Roman invasion came too late. After the annexation of Gaul a mixed Gallo-Roman culture began to emerge.
Franco-Visigothic Wars
The Franco-Visigothic Wars were a series of wars between the Franks and the Visigoths, but it also involved the Burgundians, the Ostrogoths, and the Romans. Most noteworthy war of the conflict would be the "Second Franco-Visigothic War" that included the famous Battle of Vouillé and resulted in Frankish annexation of most of Southern France.
In 486, Clovis I defeated the Gallo-Romans decisively, impelling the commander, Syagrius to flee to the court of Alaric II. Probably in 487, while Clovis was pillaging the land and besieging the cities that resisted (at least Verdun and Paris) he sent the King of the Visigoths an ultimatum: hand-over Syagrius or risk war. Therefore, instead of aiding the exile, Alaric‒reluctant to combat the Franks‒displayed cowardice and forfeited Syagrius, whom Clovis immediately executed.