Piast pyäst [key], 1st dynasty of Polish dukes and kings. Its name was derived from that of its legendary ancestor, a simple peasant. The first historic member, Duke Mieszko I (reigned 962–92), began the unification of Poland and introduced Christianity. His son, Boleslaus I, was crowned king in 1025 with papal approval. However, some of his successors did not claim the royal crown. His successors were Mieszko II (reigned 1025–34), Casimir I (reigned c.1040–1058), Boleslaus II (reigned 1058–79), Ladislaus Herman (reigned 1079–1102), and Boleslaus III (reigned 1102–38). For his four sons Boleslaus III created four hereditary duchies—Silesia, Mazovia, Great Poland (with Gniezno and Poznan), and Sandomierz. In addition, the royal throne at Kraków and the rest of the Polish territory was to be held by the oldest member of the dynasty; thus the supreme power would pass in rotation to the different branches. This law of succession caused the temporary disintegration of the kingdom. However, Casimir II (who, probably a posthumous child, was left out of Boleslaus's will) united Mazovia and Sandomierz under his power, was made duke at Kraków in 1177, and secured (1180) for his descendants the hereditary right to the kingship. Nevertheless, dynastic struggles resumed after Casimir's death (1194) and continued until Ladislaus I restored the royal authority in 1320. With the death (1370) of his son, Casimir III, the Piast dynasty ended in Poland; it was finally succeeded by the Jagiello dynasty. Another branch of the Piasts ruled as dukes of Mazovia until 1526. In 1339, Casimir III had officially recognized John of Luxemburg, king of Bohemia, as suzerain over the Piast domains in Silesia, which in the meantime had broken up into many principalities. The Silesian Piasts, as vassals of Bohemia and mediate princes of the Holy Roman Empire, retained the ducal title and continued to hold the duchy of Oppeln until 1532 and the principalities of Brieg, Liegnitz, and Wohlau until their extinction in 1675.

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