Węgier, Polak, dwa bratanki,
Tak do szabli jak do szklanki.
Magyar és lengyel jó barát,
Karddal s pohár közt egyárant.
The Pole and Magyar like brothers stand
Whether with sword or with tankard in hand.
-Polish and Hungarian Proverb[1]
It is often rare to find in European history, at least until the present day, two neighbouring countries that have consistently, for the most part at least, maintained good relations with one another. The peoples of the British Isles fought each other for centuries, as did the Scandinavians, the Germans, and the Romance speaking western countries. Some countries, like England and Portugal, have had centuries old alliances. But they are not neighbours. They did not have to live next to each other.
Poland and Hungary, on the other hand, are one of the few European nations that have maintained good relations for most of their history. Like England and Portugal, they have a long standing friendship that goes all the way back to the Middle Ages. And up until the breakup of the Kingdom of Hungary following World War I, they shared quite the long border. Of course, that border included the Carpathian Mountains, so I suppose the old adage is true: “Good fences make good neighbours.” After all, you rarely get a more impressive fence than a mountain range.
Their noble houses intermingled, and they even shared royal families. The Hungarian Angevin dynasty ruled in Poland, and the Polish Jagiellonian dynasty ruled in Hungary. Even after both houses went extinct, Hungarians and Poles continued to play positive roles in each other’s history. Hungarian noble Stephen Báthory was elected King of Poland and was one of Poland’s best; Polish general Józef Bem became a Hungarian national hero by fighting in the 1848 revolution.
However, despite this long history of friendship, Poland and Hungary were only ever united twice. Once from 1370 to 1382 under Louis I and the second time from 1440 to 1444 under Władysław of Varna. These personal unions never continued for a myriad of reasons, but an obvious one is that neither had any sons (or in Władysław’s case, any children at all).
But.
What if things had gone different? What if Poland and Hungary had stayed in personal union longer, under Louis I's heirs?

[1] Norman Davies,
God’s Playground: A History of Poland, Vol.1. (New York: Columbia University Press, 2005), 92.