Thanks for the concise replies.
I was wondering, "did Celts and Germans, as Caesar defined them, ever unite to fight, say, another Celtic tribe?" The more I research the Celts, the less I can say with certainty about them. I don't believe there was just one large group all considering them "Celts."
Episcopus, as another aside, Nietzsche's etymology of modern Italian bella, etc, "beautiful," was that it came from bellum. Perhaps war itself was seen as a thing of beauty to the Roman mind.