The Battle of Alesia or siege of Alesia was the climactic military engagement of the Gallic Wars, fought around the Gallic oppidum of Alesia in modern France, a major centre of the Mandubii tribe. It was fought by the Roman army of Julius Caesar against a confederation of Gallic tribes united under the leadership of Vercingetorix of the Arverni. It was the last major engagement between Gauls and Romans, and is considered one of Caesar's greatest military achievements and a classic example of siege warfare and investment; the Roman army built dual lines of fortifications—an inner wall to keep the besieged Gauls in, and an outer wall to keep the Gallic relief force out. The Battle of Alesia marked the end of Gallic independence in the modern day territory of France and Belgium.

The Gallic Wars were waged between 58 and 50 BC by the Roman general Julius Caesar against the peoples of Gaul. Gallic, Germanic, and Brittonic tribes fought to defend their homelands against an aggressive Roman campaign. The 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. Though the collective Gallic armies were as strong as the Roman forces, the Gallic tribes' internal divisions eased victory for Caesar. Gallic chieftain Vercingetorix's attempt to unite the Gauls under a single banner came too late. Caesar portrayed the invasion as being a preemptive and defensive action, but historians agree that he fought the wars primarily to boost his political career and to pay off his debts. Still, Gaul was of significant military importance to the Romans. Native tribes in the region, both Gallic and Germanic, had attacked Rome several times. Conquering Gaul allowed Rome to secure the natural border of the river Rhine.