The non-federated mobile field army, known as the "comitatenses", was eventually split into a number of smaller field armies: a central field army under the emperor's direct control, known as the "comitatensis palatina" or "praesentalis", and several regional field armies. Historians Santosuosso and Vogt agree that the latter gradually degraded into low-quality garrison units similar to the "limitanei" that they either supplemented or replaced. By the 5th century, a significant portion of Western Rome's main military strength lay in rented barbarian mercenaries known as "foederati".