A Messy Divorce - The Polish-Lithuanian civil war

So imagine a following scenario. The reign of King Jagiello of Lithuania goes a little bit differently. After being elected as the king of Poland he quickly earns the ire of polish nobility. Many coveted positions in the royal court are occupied exclusively by lithuanian dukes and the king seems to spend most his time in Vilnus. The king had effectively made vilnus the nations capital, a direct violation of the terms of the union (since Lithuania was to be incorporated into Poland not the other way round). The final nail in the kings coffin woukd prove to be the fated conflict with the Teutonic Order. While he suceeds in defeating the teuton forces his failure to capture Marienburg and the meager concessions made by the order would greatly tarnish his reputation. This failure to capitalize on such a victory would be highlighted by his opposition and serve to gather even more supporters to their cause. On March 15th 1412 the king is poisoned.

Message announcing the begining of a national noble gathering soon reached every noble household. Citizens from both respective countries were invited to debate on their shared fate moving forward. That at least, was the conveyed intention. During the assembly the ascendancy of a masovian duke was announced, backed by the anti-lithuanian faction. Within minutes lithuanian nobles present were disarmed and placed under arrest. This new king it seemed wanted to make due on the incorporation of Lithuania into the crown and he had no qualms about employing a direct approach. The royal army entered Lithuanian soil, intent on subjugating it. The Polish - Lithuanian war has begun. The Lithuanian Hetman Vytautas quickly assembled a force and rose out to meet his opponent. He was however only able to muster less then half the amount of polish forces. On the plains of Brest both armies tested their mettle. After a long grueling battle the polish forces came out victories. The image of the fleeing Lithuanian army inspired many notable poets and artists in the latter years, the battle becoming to this new polish society something akin to IRL's Grunwald.

After his defeat Vytautas switched tactics, abandoning the fertile Ruthenian lands. He decided his first priority is preventing the invaders from reaching Vilnus, which meant concentrating all forces in northern Lithuania. Through a combination of his excelent knowledge of the terrain and hit and run tactics he was successful in defending the capital. The war was however far from won. As the months passed it became increasingly clear that neither side had the capabilities to fully dominate one another. A settlement was reached. The treaty of Minsk stated that Poland was to incorporate all southern Lithuanian lands. The northern Lithuania known from now on as the High Kingdom of Lithuania was to retain its northern lands.

So what happens next?
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