The territory of modern Lithuania was firstly inhabited by the cord-ware culture people, and then they gave rise to the Balts. In 1230-1240s Duke Mindaugas organized the Grand Duchy of Lithuania (GDL), which included the lands inhabited by the Lithuanians, the Samogitians, the Yotvingians, and the Couranians. He also attempted to adopt Christianity. Lithuania almost doubled in size under the rule of Grand Duke Vytautas. It became one of the largest states in Europe. The country possessed present-day Belarus, most of Ukraine and the Smolensk region of western Russia. In 1569 according to the Union of Lublin Lithuania was united with Poland into a commonwealth. At the end of the 18th century most of Lithuania became part of the Russian Empire. During WWI the German army occupied Lithuania, later the independence of the country was proclaimed. Soviet domination was again restored after the Soviet-German agreement of September 28, 1939. After World War II with the purpose of integration and industrial development the Soviet workers, especially Russians, were imported into the country. Lithuania achieved complete independence after the Russian troops withdrawal during the period, called Perestroyka.