It broke the treaty.
The Demyansk Accord
The treaty that said the borders were closed unless specific permission was given by either nation for the other nation's nobles to pass through.
Look, just because your character doesn't realize Allison is scheming and manipulative and only tells your realms biased info, doesn't mean you the player has to believe that, too.
And yes, it was an actual in-game treaty too. It was replaced by the "Terran Reiteration of Sovereignty" when Kabrinskia started sending nobles in to purposefully violate the Demyansk Accord.
isn't that an accord or declaration of closed borders, not a treaty.
OK, Kabrinskia violated the closed border and that is a valid reason to declare war. but we did not break a treaty.... sorry but it is a point of honor.