The Ala Afrorum
If you go out in the woods today, and visit some of the Roman archaeology museums along the Rhine in what was once Germania Inferior, you're liable to run into tombstones of troopers of an Ala Afrorum. At first glance, that seems odd - who'd send a cavalry unit from what is now Tunisia to the swamps and forests of the Lower Rhine area, especially when excellent cavalry was available locally? After all, Celtic and Germanic cavalry had served Caesar and his heirs. Regular readers might counter that another cavalry unit, the Ala I Canafatium from what is now the area around The Hague and Rotterdam in the Netherlands, was sent to Numidia in the second century. In other words, the Romans didn't think twice about shuffling units across the empire.
Recruiting far and wide
The Ala Afrorum seems to have come from Africa just after the Batavian Revolt in AD 69/70. This revolt was the likely catalyst for a trend to move ethnic auxiliary units away from their recruiting grounds. The Batavians had a special status among subject nations, and their revolt shocked the empire. Even though the relationship was quickly reestablished, these and other auxiliary units subsequently served outside their own provinces. In the case of the Batavians, that became northern England, as well as the Balkans.
It seems that, for a while, the Romans maintained the ethnic integrity of these units. After all, the whole idea of moving these units around was to prevent local ties. That soon seems to have been forgotten, however. Auxiliary units started to recruit regionally, or even locally.
The surviving tombstones of troopers in the Ala Afrorum provide ample evidence. There's only one inscription of a (probable) trooper in the Ala who is clearly of African extraction (© Museum RGM Köln, hosted at www.manfredclauss.de).
Insert an 'H' to taste here and there, and it's obvious where "Hanno, son of Himilco" is from. For good measure, he lists himself as Afer, (from the province of) Africa. Several other inscriptions were found in Cologne, which makes it likely that Hanno served in our Ala.
The other known troopers and officers of this unit were mostly Germani. The evidence isn't always particularly 'hard', but names, tribes, affiliation and the gods they worshipped can make a strong case. The single known Praefectus of the Ala was a Roman equestrian, of course. Some of his subordinates hint at a northern extraction, however.
The name of the decurio Flavus ("blond") certainly is suggestive. Simplex son of Seplus, who made it to the governor's guard, worshipped to the Matres Masanabus. Though exactly what these specific mother-gods meant is unknown, Matres in various forms were venerated throughout northwestern Europe. The signifer Oclatius, son of Carvus, was a Tungrian, a Germanic tribe from northern Belgium that gave its name to the modern city of Tongeren. Using someone's Romanised name as evidence for their origin is considered problematic nowadays, but a few are clear enough. Marcus Traianus Gumattius, son of Gaisio, whose tombstone spent centuries after its discovery in a church spire in Dodewaard, was certainly Germanic. His own given name, Gumattius, is otherwise unknown, but Gaisio, that of his father, means something like 'spear bearer' in Germanic.
The unit seems to have stayed in Germania Inferior for most of its service, though it did pick up at least one Romanus (what's in a name?) of the Dardani, a tribe in Moesia Superior. Was the unit, or a vexillation, deployed there at some point?