Such expeditions into unknown land distant regions were not unusual for the Gauls, who marched as bands of armed emigrants, troubling themselves little as to the means of cover or of retreat; but it was evident that none in Rome anticipated the dangers involved in so sudden and so mighty an invasion.

It was not till the Gauls were marching upon Rome that a Roman military force crossed the Tiber and sought to bar their way.
Not twelve miles from the gates, opposite to the confluence of the rivulet Allia with the Tiber, the armies met, and a battle took place on the 18th July, 364.

Even now they went into battle--not as against an army, but as against freebooters--with arrogance and foolhardiness and under inexperienced leaders, Camillus having in consequence of the dissensions of the orders withdrawn from taking part in affairs.
Those against whom they were to fight were but barbarians; what need was there of a camp, or of securing a retreat?
These barbarians, however, were men whose courage despised death, and their mode of fighting was to the Italians as novel as it was terrible; sword in hand the Celts precipitated themselves with furious onset on the Roman phalanx, and shattered it at the first shock.

The overthrow was complete; of the Romans, who had fought with the river in their rear, a large portion met their death in the attempt to cross it; such as escaped threw themselves by a flank movement into the neighbouring Veii.

The victorious Celts stood between the remnant of the beaten army and the capital.