So I'm taking a course in Arthurian Legends (which, it probably goes without saying, is entirely awesome) and I came across something pretty cool while reading part of Geoffrey of Monmouth's "History of the Kings of Britain." At this point, Arthur and his troops are locked in battle with the Roman empire and their various allies. The relevant passage follows:
"First a loss was inflicted on the Britons, because Bedivere the Cupbearer was slain, and Kay the Seneschal mortally wounded. For while Bedivere was fighting Boccus, king of the Medes, he was pierced by that one's lance among the enemy troops and collapsed, slain. When Kay the Seneschal tried to avenge him, he was surrounded by the troops of the Medes and received his death wound. Yet like a good soldier, with a flank that he was leading, he opened a way through slain and routed Medes and would have retreated to his own lines with his division intact if he had not encountered the division of the King of Libya..." (The Romance of Arthur, New, Expanded Edition: An Anthology of Medieval Texts in Translation, p. 88)
So it seems, at least according to the relatively unreliable Geoffery, that the Medes were active, allied strongly with Rome, and pretty bad-ass way back in Arthur's time. (The dates of which are not easily ascertained, unfortunately.) Even if Geoffrey's Medes don't reflect reality, it's still cool to connect fiction to fiction.