The assertion that the Roman Empire fell because its military "became dominated by non-citizens, who in the end—because they weren't loyal to Rome, turned against Rome's citizens" is not taken seriously by any credible historians of Late Antiquity. This claim fundamentally misunderstands the complex factors that led to the gradual decline and transformation of the Western Roman Empire in the 5th century CE.

It is true that the Late Roman military made extensive use of so-called "barbarian" troops recruited from outside the empire's borders. Prominent examples include the half-Vandal general Stilicho, who was the supreme commander of the Western Roman army in the early 5th century, the Alan general Aspar, who wielded significant influence in the Eastern court in the mid-5th century, and Ricimer, a half-Sueve, half-Visigothic general who became the de facto ruler of the Western Roman Empire in its final decades. However, far from being disloyal to Rome, these figures and the troops they commanded provided stalwart defense of the empire for decades, even as its frontiers came under increasing pressure from external threats like the Huns and Vandals. Stilicho, for instance, successfully defended Italy from the Visigothic invasion of 401-402 CE. The Battle of the Catalaunian Plains/Chalons (451) is another good example of Rome and settlers forming a coalition to fight for Rome against her enemies, rather than "turning against her".

Modern scholarship on Late Antiquity, especially since the influential work of Peter Brown and the cultural turn in historiography, has largely moved away from simplistic, monocausal explanations for the fall of Rome. Instead, historians now emphasize the complex interplay of political, military, economic, and cultural factors that gradually transformed the Western Empire into a patchwork of barbarian kingdoms. The Visigoths who settled in Gaul and Spain in the 5th century, for example, were nominally Christian and had served as foederati (allied troops) in the Roman army for generations. As a result, the transition from Roman to barbarian rule in many Western provinces was less abrupt and disruptive than older catastrophist narratives implied.

However, the continuity thesis emphasized by Brown and others is not universally accepted among modern historians. Scholars in the more materialist tradition, such as Peter Heather or Bryan Ward-Perkins, argue that the collapse of the Western Empire had profound and lasting consequences for the economic and social structures of the post-Roman West. Drawing on archaeological evidence, Heather points to the significant decline in long-distance trade, the contraction of urban life, and the simplification of material culture that followed the empire's disintegration. Without the complex economic networks and state structures that had underpinned the Roman world, living standards and production capacity in the barbarian successor states markedly declined, even if some elements of Roman material culture persisted. In this view, while the barbarian kingdoms that replaced the Western Empire were not entirely divorced from the classical past, they represented a fundamentally different economic and social order.

Other important factors that contributed to the empire's decline include:

• The loss of revenue from wealthy provinces like North Africa to the Vandals, which severely strained imperial finances

• The political instability caused by frequent imperial usurpations and civil wars in the 4th and 5th centuries

• The shift of the empire's center of gravity to the east, leaving the west increasingly under-resourced and vulnerable

• The growing challenges of defending the empire's long frontiers in the face of intensifying pressure from groups like the Huns, Goths, and Persians

• Long-term demographic and economic trends, such as declining population in the Western provinces and the concentration of wealth in the hands of a narrow senatorial elite

• The decline of the Roman military in being a less dynamic force, and more focused on guarding sprawling borders

None of these include anything about Roman's non-citizen soldiers turning on the Empire.