The Decline of the Roman Military

[John Adams]

But weren't they hired because they were better than the Romans?

They were hired, in large part, due to the civil wars that repeatedly stripped Rome of men. As Roma no longer contained enough men to patrol the expanded frontiers, it gradually (and then steeply) turned towards Roman mercenaries, in otherwords, primarily Gemanic mercenaries, to serve as Romas' -increasingly- main armies.

Simultaniously, as Conrad points out above, while these men were being hired in increasing numbers, the Roman military machine was undergoing radical reconstruction (I would label it de-construction, as re-construction in my mind adds something positive, while this added nothing positive), to ultimately come out extremely weak and saturated with Mercenaries.

Make no mistake, Rome always had allied soldiers, and to a more limited extent, mercenaries serving in her ranks throughout history. However, as the Germanic population soared on what was once Roman soil, the Germanic influences began to wield ever increaseing power over the military machine (coupled with radical reforms under many of the Roman Emperors... I can list names if you would like).

~John