Lithuania's history dates back to the early Middle Ages when it was inhabited by Baltic tribes. In the 13th century, the Grand Duchy of Lithuania was established, becoming one of the largest countries in Europe. It expanded through alliances and conquests, notably merging with the Kingdom of Poland in the late 14th century, forming the Polish-Lithuanian Commonwealth. In the 18th century, the Commonwealth weakened and was eventually partitioned by neighboring powers, leading to periods of foreign rule. Lithuania regained independence in 1918, but was occupied by the Soviet Union during World War II. It finally restored its independence in 1990, becoming a member of the European Union in 2004.