If you asked a Roman during the Republican years to name the event in history that created the most fear in the hearts of he and his fellow citizens, the answer would undoubtedly be the Gallic invasion of 390 B.C. In that year Rome was in its one hundred twentieth year as a Republic, but her army was not large or as battle tested as it would be later.
During the spring of that year, several Gallic tribes invaded Northern Italy. One of them, the Senones, raced down the peninsula with 30,000 men in search of plunder. They rolled up towns and villages as they headed south, burning everything they couldn’t take and killing the people who had the misfortunate to be in their way. Rome did not receive word of the threat until the enemy was eighty miles from the city, but sent two legions north and set them up in battle formation at the eleventh milestone near the Tiber.
The battle was a disaster because the shear volume of men in the Gallic force was more than the defenders could handle and the Roman position was easily flanked. Half of the army was killed and the survivors retreated to Veii, on the other side of the Tiber.
Now the city was defenseless and open to plunder. The Vestal Virgins and priests gathered up the sacred relics and fled to Caere, while the remnants of the senate and army retreated to the top of the Capitoline Hill, prepared to resist any Gallic attempt to storm the citadel. The Gauls entered the city, plundered it, and set it on fire. They seemed content to let those on the Capitoline starve, and only tried to attack it on a couple of occasions. For seven months they occupied the city.
Finally, when the defenders were at a point of starvation, fortune took their side. The Gauls got word their homeland was under attack, so they decided to abandon the siege and head north. Before leaving, they demanded a thousand pounds of gold from their victims.
It took decades for the Rome to recover, and wasn’t until 378 B.C. that enough money was raised to build a wall around the city. This Servian Wall was twenty-four feet high, twelve feet thick, and took twenty five years to complete. No new enemy would enter Rome for the next 800 years.