Battle of the Allia
From Conservapedia
The Battle of the Allia was fought in central Italy between migrating Gauls and the early Roman Republic in 391 B.C. The Romans were decisively defeated and Rome was sacked by the Gauls in 390 B.C. Over time, that would prove to be a very rare occurrence.
Sources
Encyclopedia of Military History, Dupuy & Dupuy, 1979
