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 territories against external threats and to create a more centralized government. The agreement established a shared parliament and a common foreign policy, while each region maintained its own laws and customs. The Union of Lublin marked a crucial moment in European history, influencing the political landscape of Central and Eastern Europe for centuries to come.