By 1444, Poland was a decent sized country with a stable government system. The Kingdom of Poland had control of all of its modern territory except the Prussian Region and other German land controlled by the Holy Roman Empire. It also controlled Ruthenia, an area of modern western Ukraine. Diplomatically, Poland was well situated. Its current King, Władysław III, was also King of Hungary. Furthermore, despite asking the Teutonic Knights to rid them a few decades earlier, Poland had outstanding relations with Lithuania, then a massive nation in Eastern Europe. Poland also had the Duchy of Moldova as its subject; a smaller nation south of Ruthenia that acted as a buffer state between Poland and the rising Ottoman Empire. With allies to the south and east, the Holy Roman Empire remaining neutral in Polish affairs, Poland's only real enemy at the time was the Teutonic Order, an order of Knights that once fought off the pagan tribes of the Baltic region but now have declared that they will be staying.
Point of Divergence
The seemingly perfect situation all started going downhill when Władysław III was killed in the Battle of Varna, a battle that guaranteed Ottoman dominance in the Balkans. He had not married, so in result he had no heir to the throne. For the next few years, Poland was left without a ruler; it was governed by a council of nobles in recency. Desperate for a ruler, Poland offered Władysław's brother, Kazimierz Jagiellon, King of Lithuania, the throne (talk of union started in 1385, however no single individual absorbed both titles as King). Although hesitant, Kazimierz accepted, uniting the two Kingdoms of Poland and Lithuania, making it the biggest country in Europe at the time. So what was so bad about this? The reason is, when Kazimierz gained the throne, he was forced to accept a different form of government: The Sejm. The Sejm was a government dominated by nobles and gave the King very little power. Multiple reforms after each king's death by the Sejm made Poland weaker and weaker, making invasions by Russia and later Prussia and Austria very easy. As well as all these reforms, Poland became an elected monarchy, where the heir of the monarchy only could become King if the Sejm accepted him. This also allowed foreign countries to place their royalty, usually sons that were not in line for the throne of their own countries, as candidates for the Polish crown. This happened many times, and the Jagiellon Dynasty eventually died off. When foreign Kings were in power, they usually still favored their own country and allowed Poland to become subject of corrupt actions.
In this alternate history, Władysław losses the Battle of Varna but is not killed. He survives the war, and remains King of Poland and Hungary. Later on, his titles will be split and members of the same dynasty will rule Poland and Hungary separately (Austria never uses the Hungarian Regency as an opportunity to place a Hapsburg on the throne). With this major change, The Jagiellon Dynasty will rule three major eastern nations- Hungary, Poland and Lithuania- for most of its history. The alternate history aims to analyze if Eastern Europe, specifically Poland, is much stronger historically.