Ni'Var - How did the Vulcans and Romulans re-unify?

We know that in the far future of Discovery, Vulcan has been renamed Ni’Var and both Vulcans and Romulans live there. We know that this was the end-result of Spock’s efforts in Unification I and II, that they left the Federation, and that there is considerable political strife between the Vulcans and Romulans....