The Union of Lublin was a significant agreement signed in 1569 that united the Kingdom of Poland and the Grand Duchy of Lithuania into a single state known as the Commonwealth of Poland-Lithuania. This union aimed to strengthen both regions politically and militarily against external threats, particularly from Russia and the Teutonic Knights. The union established a shared parliament and a common foreign policy, while allowing both regions to maintain their own laws and customs. This arrangement fostered a period of cultural and economic growth, making the Commonwealth one of the largest and most influential states in Europe during the 16th and 17th centuries.