The Kingdom of Lithuania was a sovereign state that existed from 17 July 1251 until the death of the first crowned king of Lithuania, Mindaugas, on 12 September 1263. Mindaugas was the only Lithuanian monarch crowned king with the assent of the Pope and the head of the first Catholic Lithuanian state. The formation of the kingdom is widely regarded as a partially successful attempt at unifying all surrounding Baltic tribes, including the Old Prussians, into a single unified state under a common king.
Other monarchs of Lithuania were referred to as grand dukes, kings or emperors in extant foreign written sources as the size of the realm and their power expanded or contracted. This practice can be compared to that of British, Japanese and many other monarchs who are known as kings or emperors in spite of not being crowned with the assent of the Pope. Because Lithuania was pagan in the 13th century, Lithuanian monarchs were not granted the title of a Catholic monarch, even though extant Christian sources referred to Lithuanian rulers as kings or emperors regardless of their religious affiliation. For instance, Gediminas titled himself King of Lithuania and Rus, and Duke of Semigalia. The Pope also addressed him as King.