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.

Investment is the military process of surrounding an enemy fort with armed forces to prevent entry or escape. It serves both to cut communications with the outside world and to prevent supplies and reinforcements from being introduced.