However, the need for updated defences became acute during the crisis of the Third Century, when various tribes moved through the Germanic frontier and the Roman army struggled to stop them.

In 270, groups of Juthungi and Vandals invaded northern Italy, inflicting a severe defeat on the Romans at Placentia (modern Piacenza) before eventually being driven back.

[4] Aurelian's construction of the walls as an emergency measure was a reaction to the invasion of 270; the historian Aurelius Victor states explicitly that the project aimed to alleviate the city's vulnerability.

[5] It may also have been intended to send a political signal as a statement that Aurelian trusted that the people of Rome would remain loyal, as well as serving as a public declaration of the emperor's firm hold on power.

The root of this unorthodox practice was the imminent threat of the foreign tribes coupled with the wavering strength of the military as a whole due to being subject to years of bloody civil war, famine and the Plague of Cyprian.

The entire combined strength of the Praetorian Guard, cohortes urbanae, and vigiles of Rome was only about 25,000 men – far too few to defend the circuit adequately.

However, the military intention of the wall was not to withstand prolonged siege warfare; it was not common for the invading armies to besiege cities, as they were insufficiently equipped and provisioned for such a task.

The Aurelian Walls remain remarkably well-preserved today, largely the result of their constant use as Rome's primary fortification until the 19th century.