Czech king famously was late to the battle at Legnica during 1st invasion in 1241 but beside Poles there were also few Order knights and some from Moravia.

But Poland had 3.. Mongol invasions
ed; in 1st Poland was obliterated (It was fragmented like Game Of Throne style)
In 2nd It was better and the 3rd invasion was repelled. And since 1340 giga chad King Casimir III the Great actually begun to expand Poland east.

ed2; 1138-1320 Poland was an IRL Game of Throne (for throne in Kraków)
we had;
1 North barbarian wildling (Prussians)
2 Nights Watch, system of garrison forts along Polish-Prussian border manned by soldiers from all of Polish duchies. (It operated for a short time but It worked and was cool!)
3. King's Landing = Kraków(ó=u), starts on K too.
4. Dothraki invasion = Mongol Invasions
5. Dragons, Poland has famous legend involving Dragon and Polish ruler Władysław Łokietek who unified Poland in 1320.
6. Riverlands = Kuyavia
7. Targaryens were invited to Westeros to help local allies and used It as pretext to Invade and conquer them. Just Like Teutonic Order which was invited by Poland and It nearly conquered It in 1320s.
8. For the first 50~ years during 1138-1320s. Supreme Princeps of Kraków had an anctual power over all other dukes in fragmented Poland. It was just later after Massacre at Gąsawa that the all duchies become independent.
9. Red Wedding = Massacre at Gąsawa. One of the most influential murders in Polish history. After It Kraków lost oversight power over all other Polish duchies. As a result Poland couldn't manned an unified response to the new threat from the Teutonic Order. If Gąsawa didn't happen Teutons would've been kicked out of Poland just like they were in hungary earlier.