Why was the Grand Duchy of Lithuania one of the largest states in Europe during the late Middle Ages?

The Grand Duchy of Lithuania became one of the largest and most influential states in Europe during the late Middle Ages primarily through systematic territorial expansion and strategic political management.1

Key factors contributing to its growth included:

• Strategic Expansion: Under the leadership of Gediminas (reigned 1316–41), the state expanded its frontiers from the Baltic Sea to the Dnieper River. His successor, Algirdas, further extended these domains to the Black Sea by conquering vast East Slavic and Tatar territories.12
• Political Talent and Tolerance: Because ethnic Lithuanians were not numerous enough to colonize these vast regions, the state maintained control through political skill and religious tolerance. The ruling elite allowed the local Ruthenian nobility to retain their Orthodox religion, privileges, and local authority.12
• Military and Administrative Integration: The state reorganized its military and administration based on Ruthenian models, and the Ruthenian language became a de facto official language for administrative purposes.1
• Strategic Alliances: Under Vytautas the Great, the state reached its peak power, decisively defeating the Teutonic Knights at the Battle of Tannenberg in 1410, which permanently reduced the German threat.1