The people of Van have been horribly oppressed by the Principality, and pretty much brainwashed to think it is the Kingdom fault. King Souma here is about to show these people what Freedom actually is, until the Empire is forced to make Souma return Van to Julius.
And then Julius IMMEDIATELY returns things to the oppressive norm leading the people of Van to go "The Foreign King who invaded us treats us better then our 'rightful' ruler....". After an unrelated rebellion happens else where in the country, Julius takes his solders to go suppress it. Yes, going out to kill farmers and villagers that rebelled against a shitty local noble. (Side note, this was instigated by one of the other neighboring countries already planning to start take their own piece of the Principality)
As soon as Julius and his soldiers are clear of the city, the people of Van rise up (instigated by Souma's spies), take control of the palace, and immediately send word to the Kingdom that they want to be Re-annexed and added to the Kingdom.