It is very obscure what or who the "Mee-Dis" is or are. It is possible that the Mee-Dis is the name of an outsider people that invaded D'ni, like how the Pento War refers to the Pento people. The hyphen, or bar, might indicate an exotic or compound name for that people.
On the other hand, the bar could suggest that the war was fought between two groups, the Mee against the Dis. In any case, the nature of that/those group(s) is totally unknown.