In 410 Flavius Aetius was sent to the Huns as a hostage and the military upbringing he received was described as giving him a 'martial vigour' uncommon among other Roman Generals at the time. What is meant by 'martial vigour' and why was this uncommon among the other Roman Generals of this period?

Oh, this is my field.

Okay so let's basically give an overview of the early life of Aetius first, and then talk about your specific question. This mostly comes from Ian Hughes' Aetius: Attila's Nemesis and Meghan McEvoy's Child Emperor Rule in the Late Roman West, as well as Jeroen Wijnendaele's recent paper "The Early Career of Aetius and the Murder of Felix."

Flavius Aetius was probably born in about 391 AD in Durostorum (modern Silistra), to his father Gaudentius and his mother who had connections to the Italic aristocracy. Aetius is usually stated to be "half-Goth" but it's difficult to say whether his "Scythian" father was Goth, or rather it referred to the fact he hailed from the region of Thrace.

By about 404 AD Aetius was already enrolled in the Protectores Domestici, founded as a Palace guard unit but by this point an empty roll and placeholder for earmarked political candidates so they could say they "served in the army." He was then appointed to the role of Tribunis Partis Militaris which we don't really know the function of, but it may have been some sort of earmark for a military career. In 405 Aetius was sent to the court of Alaric the Goth, and he returned upon the death of Stilicho and the purging of his supporters in 408. Gaudentius had never had a particularly illustrious career under Stilicho, only briefly serving as Comes Africae after the revolt of Gildo in 398, but then being transferred back into obscurity. Aetius was then sent to the court of the Huns in 408 or maybe 409, where he stayed under Uldin and then Charaton. It's not known when he came back to the Roman court, but he and his father didn't see any real opportunity for advancement until Castinus came to power as manager of Honorius in 421, and the usurpation of Ioannes in 424. Aetius was appointed Cura Palatii, which actually is now thought to have been a more martial position and commander of the palace guard, while Aetius' father was killed in a revolt in Gaul after being appointed Magister Militum per Gallias, as Ioannes wasn't widely accepted. Aetius was sent to the court of the Huns to acquire a force of federates, but arrived too late to assist Ioannes against Theodosius II's campaign to place Valentinian III on the throne. When he returned, he managed to leverage his position into acquiring the command of his recently deceased father as Magister Militum per Gallias.

The phrase regarding "martial vigour" is usually recycled or paraphrased from Edward Gibbon, who wrote this about Aetius in the late 1700's when he compiled his monumental The Decline and Fall of the Roman Empire. It comes from a passage preserved via Gregory of Tours in his Decem Libri Historiarum from Renatus Profuturus Frigeridus, who wrote a history that went possibly to the death of Valentinian III but sadly does not survive. Aetius' martial traits are also described in laudatory poetry from one of his two known Panegyricists, Flavius Merobaudes (probably descended from a Frankish general of the same name).

It's often stated Aetius developed a "martial vigour" that was "lacking" at the time because of his exposure to Barbarian military culture and strategy. Aetius undoubtedly picked up a variety of skills, diplomatic contacts, and possible military experiences during his time with the courts of Alaric, Uldin, Charaton, and Rua, but to say this was somehow exclusive to him is misplaced. Roman generals certainly did not lack a "martial vigour," but the grand strategy of the Roman military was undergoing fundamental changes in the late 4th and 5th centuries. The Romans were focusing more and more on diplomacy and subterfuge, and started to see their armies as the last resort since they were costly to replace if disaster struck. Under Constantius III, the general prior to Aetius, the Roman army had campaigned using maneuver and blockade rather than set piece battles, starving out the Goths and then forcing them into a treaty and sending them against the Suebes, Alans, and Vandals in Spain, before the Romans themselves successfully followed up with a second campaign in 419.

What made Aetius different was that he was aggressive, and it's worth noting that Aetius actually never won any set piece battle he fought. All of the known descriptions of Aetius' campaigns: Mons Colubrarius in 438 and Vicus Helenae in 445, show him as a shock and ambush commander. Litorius in 437 defeated the Goths outside Narbona before they could draw up a battle line. Aetius did the same at Mons Colubrarius 438. He defeated the Goths in a night attack in 439 (after Litorius was defeated in a set piece battle outside Tolosa). He and Majorian (future emperor) defeated the Franks in an ambush on a wedding ceremony (probably literary fluff but it was an ambush on their camp) in 445. His two known set piece battles were a defeat (Ariminium in 432, where he wounded Bonifatius), and indecisive (the Catalaunian Fields in 451, which is what my book is on).

If anything it can be said Aetius clearly fought in the steppe style of warfare: utilizing the Roman army and his federate forces aggressively and avoiding set piece engagements by attacking before the enemy was able to respond. He was brilliant because he constantly maintained the upper hand over his opponents and stayed a step ahead of them in order to execute these surprise attacks, meaning he must have had an excellent intelligence network on top of a smooth logistical base.

Aetius certainly had a lasting impact in Roman propaganda, with authors like the 6th Procopius naming him "the Last of the Romans" (one of the only historical figures actually to be named such by the Romans themselves) and the contemporary Historian Priscus dedicating a significant eulogy in his fifth book to him. He was a complex individual, and likely hardly a reflection of virtue considering the political intrigue he ended up involved in, in his bid for power. He was ultimately viewed as a model of "true Roman ideals and virtue" and what have you, but in reality the idea that this "martial vigour" was exceptional to him isn't exactly true, just Roman idealism.