Slovak history is marked by its early settlement by Celtic tribes and later by the arrival of the Slavs in the 6th century. The region became part of the Great Moravian Empire in the 9th century, which was significant for the spread of Christianity and the development of the Slovak language. In the 20th century, Slovakia was part of Czechoslovakia, formed after World War I. Following World War II, it became a communist state until the peaceful split in 1993, leading to the establishment of the independent Slovak Republic. Today, Slovakia is a member of the European Union and NATO.