Celtic warfare
The "Gallic Wars" were a series of military campaigns waged by the Roman proconsul Julius Caesar against Gallic tribes, lasting from 58 BC to 51 BC. The Romans would also raid Britannia and Germania, but these expeditions never developed into full-scale invasions. The Gallic Wars culminated in the decisive Battle of Alesia in 52 BC, in which a complete Roman victory resulted in the expansion of the Roman Republic over the whole of Gaul. The wars paved the way for Caesar to become the sole ruler of the Roman Republic.
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.