Lithuania's history dates back to the formation of the Grand Duchy of Lithuania in the 13th century, which became one of the largest countries in Europe. It reached its peak in the 15th century, expanding its territory and influence through alliances, including the Union of Krewo with Poland in 1386. In the late 18th century, Lithuania was partitioned by neighboring powers, leading to a loss of independence. It regained sovereignty in 1918, but was occupied by the Soviet Union during World War II. Lithuania declared independence again in 1990, becoming a member of the European Union in 2004.