The traditional account of the martyrs of the Theban Legion has long been regarded as historically unreliable. However, more modern research has revealed that it might be more reliable than we thought. This was demonstrated by Prof. Donald O’Reilly in an article published in Vigiliae Christianae in 1978, “The Theban Legion of St Maurice,” followed by his book Lost Legion Rediscovered: The Mystery of the Theban Legion. (Pen and Sword Military, 2011)
Would Rome really send a legion 3,500 miles away?
Some people have raised the objection against the story that there was no reason why a legion assembled in Egypt should be sent so far away as the area around Lake Geneva, where the martyrdom took place.
This assumption is clearly incorrect prima facie; Roman legions were often moved considerable distances for all sorts of reasons. Such a move becomes all the more likely in a period like the early reign of Diocletian and Maximian. The two co-emperors were literally pulling the empire back from the brink of destruction, and it is reasonable to assume that they might well have needed to bring in fresh troops from afar to deal with one crisis or another.
Other clues
Prof. O’Reilly makes several very interesting observations regarding some of the other historical difficulties of the story. One refers to a papyrus dated to the year 282 and found at Panopolis, which is not far from Thebes, the ancient Egyptian city where the legion was recruited. This papyrus records the requisition of a quantity of bread large enough to support a legion-sized unit for three months, roughly the time needed to travel at a military march from Egypt to Gaul. In the same period, coins were minted in Egypt of a type specific to the commemoration of the founding of a legion. This proves that a legion was in fact recruited in Egypt and then sent somewhere very far away.
He then argues that the principal objection to the legend, the massacre of an entire legion of over 6000 men, is also the result of a misunderstanding.
Among the many things that Diocletian did to resolve the long-standing crisis of the third century, he effected a major reorganization of the Roman army, in which many legions were brought down to only 1000 members. By 293, we have a document called the Notitia Dignitatum, a detailed explanation of the Roman imperial administration which includes the names of many military offices and titles. This document lists four such units as the bodyguard corps of the four co-emperors (or “tetrarchs”); each unit was named after one of them (e.g. “Legio Diocletiana”), and qualified with the words “Thebaeorum – of Thebans.”

Thinking of the fate of their predecessors over the last sixty years, almost all of whom were murdered by their own troops, what better guards could the Tetrarchs find than the members of a predominantly Christian legion, men who believed, as a matter of strongly held religious conviction, that assassinating an emperor, allowing his assassination, or conniving at it, would be a grave offense against God?
If we suppose that Diocletian and Maximian took the members of a predominantly Christian legion as their bodyguards, the original Theban legion would therefore not have been massacred to a man for their Christian faith.
Rather, after suffering a decimation (and that, very possibly for some matter having to do with their religion), they were simply organized out of existence as a unit, and its former members were assigned to the four newly created corps of imperial bodyguards. This fact would then later have been forgotten in the light of the fact that these same two emperors would later stir up the last and greatest ancient Roman persecution of the Christians.