Only the Gauls had managed that

The Republic had no fiercer bogeyman than the pale-skinned, horse-maned, towering Gaul. Hannibal might have ridden up to Rome's gates and flung his javelin over them, but he had never succeeded in capturing the seat of the Republic. Only the Gauls had managed that. Way back, at the beginning of the fourth century BC, a barbarian horde had burst without warning across the Alps, sent a Roman army fleeing from it in panic, and swept into Rome.

Tom Holland, Rubicon, 234.