What was the greatest failure of the Roman Empire?

What were the three biggest defeats of the Romans?

Battle of Alia (390 BC)

An almost forgotten war, a forgotten war between the rising Roman Republic and the invading Gaul tribes under their chief, Brennus.

The battle was notable because the defeat of the Romans led to the first looting of the Romans. A humiliating defeat humbled the Romans, almost succumbing to the Gauls and their Latin allies.

But in this defeat helped the Romans rise from the ashes. Their army has improved. Discard phalanxes into something similar to the legion system. The Latin Confederates, who abandoned them in battle, were cornered. Their rivalry with the Gaulish tribes paved the way for hatred and near-genocide, which eventually led to Julius Caesar conquering all of Gaul centuries later.

2. Battle of Carré

The battle between Crassus and the Parthian army under Surena was decisive and one-sided for the Sassanids.

Despite its large numbers, Surener was able to defeat Crassus's legions with a full cavalry force of 90% mounted archers and 10% cavalry.

The Romans lacked both capable cavalry detachments and effective range weapons (i.e., javelins) to compete with rain of arrows and devastating armored catapults.

The defeat of Rome brought shock waves to the Empire. The loss of the legions led to the supremacy of Parthian supremacy in the east, while the death of Crassus ended the Roman Triumvirate. This led to inevitable clashes with the remaining two parties under the leadership of Caesar and Pompeo, which would pave the way for the fall of the republic and the rise of the Roman Empire.

3. Battle of the Teutoburg Forest

Arrogance seems to be a general trend in Rome's failure.

Nothing says more than Quintus Varus's defeat at the Battle of the Teutoburg Woods and the destruction of 3 entire legions. I say the battle is because of the many conflicts between the Romans and the Germanic tribes in the forest, never completely overtly fighting.

This failure prevented Rome's expansion into Germania. Putting an end to Rome's dream of expanding beyond the Rhine and providing breathing space for Germanic tribes would soon lead to a great migration of tribes, eventually culminating in the decline of the Roman Empire centuries later