In 378 the eastern emperor Valens fell, along with many of his officers and two- thirds of his army, fighting against a Gothic army at Adrianople. It was a major disaster, but its consequences should not be exaggerated. The defeat was largely the result of Valens’ mistakes and not of the army’s inefficiency, although it may have reflected the unpreparedness of the fourth-century army for operating in large numbers. After the battle the Romans reverted to the low-level harassing operations in which the fourth-century army excelled, and by 382 the Goths accepted peace terms and provided troops for the Roman army in subsequent years. Adrianople was a setback and a significant blow to Rome’s prestige, but it was not an indication of a decline in quality of the army. At the end of the fourth century it was unlikely that anyone could have imagined that the Empire would ever cease. Certainly the tribal peoples which forced their way through the frontiers in the west did not aim to destroy Roman power, but to create a favourable place for themselves within the Roman system.
Potentially the army of the late Empire was as efficient a fighting force as any earlier Roman army. Its professionalism, discipline, training and good equipment, supported by a well-organized logistical system, gave the Roman army significant advantages over any opponent. Well led, a Roman army was able to defeat larger enemy forces. Not all units of the army reached the highest standards of discipline and confidence in the fourth century, but this had also been true in earlier centuries. It took long years of successful campaigning to raise troops to the peak of efficiency. The fourth-century army lacked some of the flexibility in fighting different scales of warfare that had marked the army of the Principate. It excelled in lower-level operations, but its unit and command structure was less suited to large battles. This was a reflection of the raids and skirmishing that were the most common types of combat experienced by the late Roman soldier. A reluctance to escalate a conflict and reach a swift decision by defeating the enemy in battle was a marked change from earlier practices. Roman warfare lacked something of the relentless quality which had distinguished it at earlier periods. Far more conflicts ended in treaties that did not give Rome total victory.
However, while the later army had the potential to be very efficient it was able to fulfil this potential less often. Its command structure was heavily divided at all levels, from the imperial power downwards. This made it difficult to co-ordinate operations on an Empire-wide basis. The limitanei were tied to distinct regions which suffered if they were removed and even the comitatenses gradually became divided into increasingly numerous regional field armies. The large number of duces and comites, and the division between civil and military administration, often made it unclear who was responsible for dealing with problems on the frontiers and slowed the process of mustering and supplying an army. Emperors found themselves having personally to direct very small-scale operations as the only way of ensuring that something happened. Frequent civil wars, the only conflicts in which battles were common, wasted the strength of the army in costly campaigns, while denuding the frontiers of troops and allowing external threats to grow. The only guarantee of power became the army, but the closer and more immediate link between soldiers and emperors only increased the chance of usurpation. The weakness of central authority encouraged the development of regional power, which could only be maintained by the presence of troops. There were probably more soldiers under arms in the late Empire, but it was far harder to amass armies of more than twenty thousand, and dangerous to involve them in conflict for too long in any one area. Expediency, especially during civil wars, encouraged emperors to seek recruits wherever they could be found and sometimes led to unwise concessions to tribes.
The Roman Empire did not fall quickly despite all these internal weaknesses. Its own strength, derived from the successful absorption of so many peoples and the prosperity which it had brought to the provinces, was still great. Added to which, the external threats it faced were always sporadic, disunited and weak. Political divisions weakened the Roman army, but it still proved capable of winning most of the wars it was called upon to fight. Civil wars sapped Rome’s military strength, but their frequency was a direct result of the failure of central authority to control the army.