The 390 BCE battle at the Allia River was fought between the city state of Rome and Gauls from northern Italy. When the Gauls laid siege to the Etruscan city of Clusium, the Romans intervened on behalf of the latter. The Gauls withdrew but returned to advance on Rome itself. Close to Rome, at the Allia River, the Roman army met the Gauls in battle but suffered a crushing defeat.
This is a companion discussion topic for the original content at https://www.worldhistory.org/article/879/the-battle-at-the-allia-river-390-bce/.