It was his father, Prince Mieszko I who accepted Christianity as the religion of the realm in 966 AD, who is considered to be the founder of Poland and the Piast Dynasty. But it was the son, Boleslaw I popularly known as Boleslaw the Brave, who really created the Polish state.

King Boleslaw I transformed his small "Polish" duchy from a principality consisting primarily of Wielkopolska ("Greater Poland" along the Warta River) into a true sovereign kingdom.  He firmly stood eyeball to eyeball with the Holy Roman Empire and didn't blink. He insisted that the Catholic Church in Poland be independent of German control.

Then he surveyed the area and began adding lands inhabited by other West Slavic peoples to his realm. Malopolska and Silesia were among the first. In 1003 Moravia and Slovakia, in 1007 Pomerania, in 1018 Grody Czerwienskie or Red Ruthenia which included Sanok, Przemysl, Belz, Czerwien, and Wolyn. For a time around 1003-1004, he also ruled Bohemia.

In 1018 at the request of Sviatopolk I, he led a military expedition to Kiev, capital of the Kievian Rus, to assert Sviatopolk's claims. That same year, the terms of the Peace of Bautzen settled the possession of Meissen and Lusatia in Polish favor. Today, although citizens of Germany, the West Slavic Sorbs of Lusatia consider King Boleslaw the Brave to be a national hero.

The remarkable nation-building efforts of King Boleslaw the Brave were largely undone in 1138 when King Boleslaw III "the Wrymouth" divided the country into five principalities for his sons: Krakow, Sandomierz, Mazovia, Silesia, and Greater Poland. It would take two hundred years and another equally talented king, Kazimierz Wielki, to put the pieces back together again.