Poland's Piast dynasty united the West Slavic tribes on present day Poland's land into a common nation. It's princess and later kings expanded it's borders westward and eastward. By the end of the last and most famous Piast's Casimir the Great's rule, Poland had become a rich, modern in comparison with its contemporaries, and respected country in all of Europe.
en.wikipedia.org/wiki/History_of_Poland_during_the_Piast_dynasty
en.wikipedia.org/wiki/History_of_Poland_during_the_Piast_dynasty