The fact that Poland and Lithuania merged voluntarily and fraternally was of vital importance, as evidenced by the Manifestation of the Unity of the Polish-Lithuanian Commonwealth in Horodło. Although held in 1861, as Poland was under partitions at that time, the gathering was attended by 10,000 participants from all over the country. Poland, 1634. Poland's territory in 1634, during the reign of Władysław IV Vasa. The dual Polish-Lithuanian state, Respublica, or "Commonwealth" (Polish: Rzeczpospolita), was one of the largest states in Europe. The Polish-Lithuanian Commonwealth started as a "personal union" of the King of Lithuanian in 1386, when Jagiello married the Polish queen Jadwiga (who died in childbirth). That is, the same "king," ruled both countries, separately, not as a unified country. Union of Lublin, (1569), pact between Poland and Lithuania that united the two countries into a single state. After 1385 (in the Union of Krewo) the two countries had been under the same sovereign. But Sigismund II Augustus had no heirs; and the Poles, fearing that when he died the personal union. Poland and Lithuania established diplomatic relations from the 13th century, after the Grand Duchy of Lithuania under Mindaugas acquired some of the territory of Rus' and thus established a border with the then-fragmented Kingdom of Poland. Origins Poland and Lithuania had close political ties before the 16th century. In fact, there had been a type of union between the two states since 1386 through royal marriages. However, the The Polish-Lithuanian union was a relationship created by a series of acts and alliances between the Crown of the Kingdom of Poland and the Grand Duchy of Lithuania that lasted for prolonged periods of time from 1385 and led to the creation of the Polish-Lithuanian Commonwealth, or the "Republic of the Two Nations", in 1569 and eventually to the This later led to the creation of much more unified Polish-Lithuanian Commonwealth in 1569. The main reason why the union was brockered in the first place, was the common threat that both Lithuania and Poland faced, in the form of aggresively expanding Teutonic Order. FBZPPT.