Joint Polish-Lithuanian political agreements, which were the next stages of crystallization of the Commonwealth of Both Nations have developed the concept of the Jagiellonian idea. The definition is not uniform and is sometimes interpreted differently. The project of the united Polish-Lithuanian state is present in a number of studies and assessments. The Union of Lublin concluded between Poland and Lithuania gave the foundation a strong assertion of the possibility of harmonious cooperation between the two nations in one state. Although originally developed for military purposes, it quickly transformed into a bridge, over which roamed the armies, merchants, craftsmen, priests, scholars, nobles, and illiterate populace of both nations. The two countries were united by the political agreement and given the opportunity of practically unlimited contacts at all levels of social activity: political, cultural, spiritual, economic. The flourishing of Lithuania since the mid-sixteenth to the mid-seventeenth century is largely associated with the aftermath of the declaration of the union with the Polish Crown. The subsequent collapse of Lithuania’s statehood and dissection of the country by Russia and Prussia for a while effectively stopped the desire to maintain a closer Polish-Lithuanian union. Only the Lithuanian national revival in the second half of the nineteenth century brought back the ideas of unification projects, which began during the rule of the Jagiellonian dynasty.