Mindaugas (German: Myndowen, Latin: Mindowe, Old East Slavic: Мендог — Mendog, Belarusian: Міндоўг or Mindoūh, c. 1203 – autumn 1263) was the first known Grand Duke of Lithuania and the only King of Lithuania. Little is known of his origins, early life, or rise to power; he is mentioned in a 1219 treaty as an elder duke, and in 1236 as the leader of all the Lithuanians. The contemporary and modern sources discussing his ascent mention strategic marriages along with banishment or murder of his rivals. He extended his domain into regions southeast of Lithuania proper during the 1230s and 1240s. In 1250 or 1251, during the course of internal power struggles, he was baptised as a Roman Catholic; this action enabled him to establish an alliance with the Livonian Order, a long-standing antagonist of the Lithuanians. During the summer of 1253 he was crowned King of Lithuania, ruling between 300,000 and 400,000 subjects.
While his ten-year reign was marked by various state-building accomplishments, Mindaugas's conflicts with relatives and other dukes continued, and Samogitia (western Lithuania) strongly resisted the alliance's rule. His gains in the southeast were challenged by the Tatars. He broke peace with the Livonian Order in 1261, possibly renouncing Christianity, and was assassinated in 1263 by his nephew Treniota and another rival, Duke Daumantas. His three immediate successors were assassinated as well. The disorder was not resolved until Traidenis gained the title of Grand Duke c. 1270.