why did poland and lithuania unite

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.
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.
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