The rejoining of Ni'Var, the united world of Vulcans and Romulans, to the Federation, agreed after a compromise brokered by Michael Burnham.
Ni'Var, formerly Vulcan, had left the Federation after the Burn. Its people, now united Vulcans and Romulans, blamed their own SB-19 project for the disaster until Discovery found the real cause. Talks to rejoin followed.
At the last moment President T'Rina asked for a clause letting Ni'Var leave whenever it chose. Burnham proposed a committee that would review all Federation members independently and offered to sit on it herself. T'Rina and Federation President Laira Rillak accepted, and Ni'Var rejoined.