Imaginiferi (1st/2nd c AD)

[M. CVRIVS ALEXANDER]

Alexander, I second Jasper's view of Tacitus being somewhat familiar with military facts. At least his father-in-law was an acknowledged commander and he himself must have served some time before becoming a senator. He might have confused something, anyway. But he definitely had insights.

Having relatives in the army does not necessarily add to one's personal expertise. As to his military service in his public career - according to what I read, he likely served, but "must have served" might be too strong an expression.