The onset of the War, actually led by conservative factions, thought to prove true the words of Kerath. Outside factions would invade with almost succeed to destroy the Ink-Making Guild and Book-Making Guilds. By the end of the War, most came to follow the teachings of Gish and thus believe in the end of most outsider involvement.
It is very obscure what or who the "Mee-Dis" is or are. It's possible that the Mee-Dis is the name of an outsider people tha 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.