to be precise- not part of Poland but, under cultural and linguistic influence of Polani- tribe around which later Poland was formed.
Warlords of Polani ruled Panonia (land of Panovi / land of masters / land of nobles) back in Proto Slavic antiquity, in time of Sarmatia when Sarmatian name was still universal name of all Proto Slavs from Balkan to Baltic, from west of Europe and British islands to the Ukrainian steppes and further. Panovi of Polani tribe ruled region (Panonia) in their struggle to control fertile soil.
Then, Romans pushed Polani influence out of Panonia but themselves kept the name for the region (Roman province) Panonia. Sarmatians/Slavs of Panonia were butchered and hunted by slave hunters and only in short period of time during Svatopluk`s Great Moravia, Panonia came back to Slavic rule. Ultimately, due to constant Teutonic and Hun invasions, Panonian Slavs were devastated and mostly assimilated in newly created Hungarian and Austrian states. Some of Panonian Slavs joined with other Slavic neighboring tribes.
Crow, with your vast knowledge you should rewrite these Wikipedia articles, because now they all tell lies: