Battle of the Arar

The Battle of the Arar was fought between the migrating tribes of the Helvetii and six Roman legions under the command of Gaius Julius Caesar in 58 BC. It was the first major battle of the Gallic Wars and ended in a tactical victory for the outnumbered Roman army.

When Caesar became governor of the provinces given to him by the Roman Senate, three of his four legions were in northeastern Italy guarding against potential threats from Thracian tribes. The Transalpine Gaul Province, however, was guarded by a single legion and exposed to invasions by the enemies of Rome.

Wikipedia