Their entire country is a piece of ****. Heres a simple summation of why this whole war is going on:

MCs country (the kingdom) has local Lords each with their own territories, just like with MCs father, and the stupid princes friends families.

One of the Lords back then decided to get greedy and so declared his own territory as his own independent Dukedom, THEN he raided and massacred (everybody, including innocent people) all nearby territories next to his own so he can steal their floating lands and crystals for his own powers growth.

The Kingdom obviously cannot allow such terrorism and invasion to happen so they struck back against the Lords forces and the Lords self-proclaimed Dukedom lost.

As reparations for all the damage hes done, the Lords country is charged with debt to repay for all of it. Also, to prevent another incident like this, a new Lord territory was stationed next to the Dukedom. This is the birth of the family of one of the stupid princes friends.

Since the Lord of the Dukedom is greedy af, they cannot accept the fact that they lost and failed to take over the country, so they instead decided to brainwash all the people still in the Dukedom with re-education instead. They teach everybody there--starting from childhood, that the Kingdom is greedy and is stealing money and resources (the reparations the f*cker owes the Kingdom basically) from the people of the Dukedom because they are plain evil as a country using the kingdoms military might to threated the Dukedom, so they have no choice but to pay. But its all a lie. They got greedy, killed a lot of people, now they have to pay for their crimes even if they dont want to. Period.

Years upon years pass with the Dukedom constantly trying to undermine the Kingdom and we reach the present.

In other words, their entire Dukedom is like that. Full of greedy, stupid, and brainwashed people who create problems on their own and then when the consequences come to bite them in the face, they blame somebody else for the consequences of their own actions.