I just started a new game last night as the Native Americans. I got the Vedic barbarian random event, and thought, "Wow, this can't be good."
My exploring warrior had gotten experience from a hut, and I had promoted him to Woodsman II for exploring speed, so I moved him onto a wooded hill to see if I could see them, or at least slow them down as they killed him. It turns out I moved him right next to the archers, who by some miracle, went the opposite direction instead of attacking him. I promptly renamed him Bear Face because he is so scary.
Six turns later I got the message, "The Celtic civilization has been destroyed." Poor Brennus, I never even got to meet him.