“The Empire” was defeated in Episode 6: Return Of The Jedi, was it not? Thereby making “The Rebels” (Go UNLV!) no longer rebels...but the de facto ruling class at that time? N’est ce pas?
That was in the old “Expanded Universe” storylines, in the books and stuff. The rebels became the “New Republic”.
That all got tossed out when Disney bought the property though, so none of it is canon anymore. Now it seems like they are going with the idea that the universe just kind of fragmented after the Empire fell, and the First Order is trying to reunite the pieces into a new empire.