The Kingdom of Poland (Polish: Królestwo Polskie; Latin: Regnum Poloniae) and the Grand Duchy of Lithuania joined in a personal union established by the Union of Krewo (1385). The union was transformed into a closer one by the Union of Lublin in 1569, which was shortly followed by the end of the Jagiellon dynasty, which had ruled Poland for two centuries.
In the Baltic Sea region, Poland maintained an ongoing conflict with the Teutonic Knights. The struggles led to a major battle, the Battle of Grunwald of 1410, but there was also the milestone Peace of Thorn of 1466 under King Casimir IV Jagiellon; the treaty created the future Duchy of Prussia. In the south, Poland confronted the Ottoman Empire and the Crimean Tatars, and in the east helped Lithuania fight the Grand Duchy of Moscow. Poland's and Lithuania's territorial expansion included the far north region of Livonia.
In the Jagiellonian period, Poland developed as a feudal state with a predominantly agricultural economy and an increasingly dominant landed nobility. The Nihil novi act adopted by the Polish Sejm in 1505 transferred most of the legislative power in the state from the monarch to the Sejm. This event marked the beginning of the period known as "Golden Liberty", when the state was ruled by the "free and equal" members of the Polish nobility.
Protestant Reformation movements made deep inroads into Polish Christianity, which resulted in unique policies of religious tolerance in the Europe of that time. The European Renaissance as fostered by the late Jagiellonian Kings Sigismund I the Old and Sigismund II Augustus resulted in an immense cultural flowering (see Renaissance in Poland).