While Souma is setting the stage for the Kingdom to take Van (and JUST Van), Roroa is orchestrating her own power play.
At the same time the Kingdom's troops were marching back to reclaim Van (they didn't go all the way home since Souma planned to keep the city in the end, after all) EVERY other city/territory of the Principality, at the same time (except the one Julius was suppressing) also had some sort of uprising or succession from the Principality and requested the Kingdom Annex them.
Yep. While Souma only wanted Van because anything more would be too much of a headache and pain to deal with right now on top of all of his other reforms and plans, Roroa was like "Nah Darlin, you're taking the whole country!"
Empress Maria was not happy about all this and kinda felt betrayed (The Kingdom and Empire became Secret Best Friends right before this), but softened when Souma explained he never intended to on taking the whole contry and expressed his fustration over now having a second country to rebuild.