In 1569, by the Union of Lublin, the dynastic link between Poland and Lithuania was transformed into a constitutional union of the two states as the Polish-Lithuanian Commonwealth. At the same time, the greater part of the Ukrainian territories was detached from Lithuania and annexed directly to Poland.
ROBERT FROST IN LUBLIN CASTLE on 1 July 1569, Sigismund August, king of Poland and grand duke of Lithuania, formally enacted an act of union bringing his two realms, joined since 1386 in what the textbooks call a loose personal union, into a closer relationship with a common Sejm (parliament) and a common council.
'Lithuania, my country!' - these words were written by Adam Mickiewicz, a Polish poet and independence activist, in the first half of the 19th century to refer to his homeland. While today, despite much political turmoil and after many years, similar exclamations can still be heard from the 200,000 or so Poles living in Lithuania.
The Polish-Lithuanian Commonwealth started as a "personal union" of the King of Lithuanian in 1386, when Jagiello married the Polish queen Jadwiga (who died in childbirth). That is, the same "king," ruled both countries, separately, not as a unified country.
Poland and Lithuania have been linked together in this history because for 400 years (from the end of the 14th century to the end of the 18th) they were united - at first by a personal union under the king, and then by a full political union. As far as practicable this history is confined to that of Poland and Lithuania.
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why did poland and lithuania unite