Fezzan has a lot of leverage in this plot. They are needed to execute almost the entire scheme to work: kidnapping of the Kaiser, shipping him to the Alliance and forcing the Alliance to shelter the exiled monarch (easily fixed by a certain chairman). Fezzan can still easily betray the Empire if it so chooses. They can leak the news to the citizens that Reinhard allowed their Kaiser to be kidnapped to justify taking the empire to war, and Fezzan can switch their Alliance (pun intended) to fight the Empire. Fezzan + Alliance is enough to stalemate the conflict. Overall, this makes Fezzan the kingmaker in this universe. Reinhard must have saw this threat and tried to recapture the Iserlohn corridor, so that the Empire can invade the Alliance through that route. A pity that operation failed spectacularly and now Reinhard is forced to cut a deal with Fezzan to subjugate the universe.