These had been in constant contact and intermittent warfare with the Empire since its northern borders were stabilized in the reign of Augustus in the early 1st century.
Although the literary sources mention laeti only from the 4th century onwards, it is likely that their antecedents existed from as early as the 2nd century: the 3rd-century historian Dio Cassius reports that emperor Marcus Aurelius (ruled 161–180) granted land in the border regions of Germania, Pannonia, Moesia and Dacia, and even in Italy itself, to groups of Marcomanni, Quadi and Iazyges tribespeople captured during the Marcomannic Wars (although Marcus Aurelius later expelled those settled in the peninsula after one group mutinied and briefly seized Ravenna, the base of the Adriatic fleet).
Indeed, there is evidence that the practice of settling communities of barbari inside the Empire stretches as far back as the founder-emperor Augustus himself (ruled 42 BC – 14 AD): during his time, a number of subgroups of German tribes from the eastern bank of the Rhine were transferred, at their own request, to the Roman-controlled western bank, e.g. the Cugerni, a subgroup of the Sugambri tribe, and the Ubii.
[6] In 69, the emperor Otho is reported to have settled communities of Mauri from North Africa in the province of Hispania Baetica (modern Andalusia, Spain).
[8] There is also doubt about whether the terms governing laeti were distinct from those applying to gentiles ("natives") or dediticii ("surrendered barbarians") or tributarii (peoples obliged to pay tribute).
[9] Reproductively self-sufficient groups of laeti (i.e. including women and children) would be granted land (terrae laeticae) to settle in the empire by the imperial government.
It has been calculated that in the Julio-Claudian era, as many as half of all Batavi males reaching military age were enlisted in the Roman auxilia.
[13] The traditional view of scholars is that the praefecti laetorum or gentilium mentioned in the Notitia were each in command of a regiment composed of the laeti ascribed to them.
[15][17] The main support for this view is a decree of 400 AD in the Codex Theodosianus which authorises a magister militum praesentalis to enlist Alamanni and Sarmatian laeti, together with other groups such as the sons of veterans.
[15] In this case, the praefecti laetorum/gentilium may have been purely administrative roles, especially charged with ensuring the full military levy from their cantons each year.
It must be treated with caution, as many sections are missing or contain gaps, so the Notitia does not account for all posts and commands in existence at the time of compilation.
The list of praefecti laetorum in Gaul contains prefects for the Lingones, Nervii and Batavi: but these tribes had been inside the empire since its inception under Augustus.
Alternatively (or additionally), they may have been descended from Germans settled in Pannonia following Gallienus's treaty with King Attalus of the Marcomanni in AD 258 or 259.
This, together with the large numbers of military units with barbarian names, gave rise to the "barbarisation" theory of the fall of the Roman empire.
[11] Instead, the evidence points to the conclusion that laeti were a crucial source of first-rate recruits to late Roman army.