The Danteri Empire occupied Xenex for over three hundred years until the planet was liberated, thanks to revolutionary leader Mackenzie Calhoun. During the conflict, the Federation attempted to intervene with peace talks, on behalf of the Danteri, who were being considered for Federation membership.
By the 2370s, the Danteri were indeed Federation members, after the rebellion of M'k'n'zy of Xenex led the Danteri to abandon Xenex in negotitations chaired initially by Starfleet Captain Jean-Luc Picard.
The neighboring Thallonian Empire fell in the year 2373, due in part from the machinations of the Danteri, who indirectly engineered a coup on Thallon that left most of the Thallonian royal family dead. (NF novel: House of Cards)