4 – Battle of the Allia (390 BC)
Although it occurred relatively early in Rome’s history, the loss at the Allia remained as one of the most embarrassing losses ever suffered by the proud republic. Not only because of the defeat on the battlefield, but also what happened next. The Senones were a Gallic tribe that invaded Northern Italy in the early 4th century BC. Livy claims that in 390 BC, the Senones were called to the city of Clusium by one of its leading citizens. He apparently wanted to gain revenge over another man who had debauched his wife. The Clusians were concerned by the appearance of the Gauls and requested help from Rome.
Initially, it appeared as if everything would die down because Rome made an agreement with the Senones. However, a quarrel broke out during the negotiations, a skirmish occurred, and one of the Senone chieftains was killed by a Roman ambassador, one of the Fabii brothers. The Gallic tribe demanded that Rome hand over the Fabii. When their request was refused, they decided to march on the city which was around 80 miles from Clusium.
As they approached the city, they were met by a Roman army near the Allia River. The Roman army numbered anywhere from 15,000-40,000 but was almost certainly outnumbered by the Senones who had anywhere from 40,000-70,000. Brennus led the Gauls and even though the Romans expanded both flanks, they still held a smaller line than the enemy. Their reserve was stationed on a hill on the right of the line. Brennus decided to attack the reserve because he thought the Romans were hiding part of their army. He believed that if he attacked the main line, his army would be surrounded.
The Romans panicked when they saw their reserves getting attacked and their left wing fled to Veii while the right wing returned to Rome but stationed themselves in the Citadel. As a result, the uncompleted walls of Rome were completely undefended, and the city was at the mercy of the Senones. Although Rome probably didn’t sustain massive losses at the Allia (though some sources say it lost two-thirds of its army), it suffered the indignity of being sacked by Brennus and his men.
The Senones marched into the city unopposed but soon turned their attention to the Citadel. After a siege, the defenders realized they were running out of supplies and were forced into a humiliating bargain. They paid the Senones 1,000 pounds of gold to leave. The Gauls were happy to accept as they had lost a lot of men due to disease. In reality, Rome recovered from the loss quickly as there is no archaeological evidence to suggest the Gauls burned the city or destroyed it in any way.
Perhaps the most significant thing about the loss at the Allia was the reaction of the Romans. Instead of accepting the defeat and moving on, they completely overhauled the military. Camillus began a series of reforms that were refined even further during the Samnite Wars later in the 4th century BC. The stiff and rigid phalanx was replaced by a more mobile unit, and the Romans began throwing javelins at enemies before attacking with short swords in hand-to-hand combat. Also, the scutum replaced the round shield. When former enemies such as the Etruscans saw their rival humbled, they reopened old wars. In one battle, Rome had lost what had taken over 100 years to accumulate.