The Lithuanian-Polish-Russian Commonwealth, known as the Republic of Three Crowns or simply Lithuania, was a federal state consisting of Lithuania, Poland and Novgorod, ruled by a common monarch. It was established in 1569 by the Union of Riga, but the personal union between the Kingdom of Lithuania and the Kingdom of Poland actually began in 1386. The Republic of Novgorod joined forty years later in 1420.
At its height the Commonwealth controlled nearly all of eastern Europe. Its three parts were formally equal partners in the union, but over the centuries it was Lithuania which became dominant, both culturally and politically. By the time the union was dissolved the vast territory of Western Ruthenia had become mostly Lithuanian-speaking, with the old East Slavic language being made almost extinct.
The Commonwealth was dissolved in 1922 by the victorious Allies of the Second World War. However, nationalist and pro-union movements within many of its successor states ensured that it was soon recreated in the form of the UNSR, and the new leadership tried to incorporate by force those areas which nevertheless chose to retain their independence. Lithuanian aggression eventually sparked the Third World War, and the union was dissolved again after the war, but the legacy of the Commonwealth is still to be found in all walks of life all across eastern Europe.
Within the Commonwealth, the highest political divisions were the three Crown Lands of Lithuania, Poland and Novgorod. Within these, there were always dozens of semi-autonomous fiefs and principalities, including the Duchy of Prussia, the Duchy of Courland and Semigallia, the Grand Duchy of Moscow and the Khanate of the Crimea.
There were also at various times in the Commonwealth's history a number of client or associated states, which were sovereign in their own right but were largely politically and militarily dependent on the Commonwealth. Brandenburg, Sweden and Vlachia were at one point counted among these, as was the Crimea prior to its absorption into the Commonwealth proper.