The reforms of the Great Sejm (1788–1792) and the adoption of the May 3, 1791 Constitution were pivotal moments in the history of both Poland and Lithuania, marking a final, ambitious attempt to revitalize the crumbling Polish-Lithuanian Commonwealth. The reforms sought to address long-standing internal weaknesses, including political fragmentation and the destabilizing influence of foreign powers. For Lithuania, these reforms were particularly significant because they aimed to integrate the governance of the Grand Duchy with that of the Polish Crown, while still preserving the distinct identity and status of the Lithuanian nobility.
The Constitution of May 3rd abolished the liberum veto—a parliamentary rule that had paralyzed decision-making—and established majority voting in the Sejm. It also reformed the decentralized state structure by creating a more unified central government, combining Poland’s and Lithuania’s treasuries and military under a shared administration. Lithuania's participation was further ensured by reserving half of the top government offices for Lithuanian nobles. This restructuring offered hope that the Commonwealth could better defend its territories, especially from the growing threats posed by Russia and Prussia.
However, the reforms, while progressive, were ultimately short-lived. The changes alienated many conservative nobles, particularly those who sought to maintain their autonomy, leading to internal resistance. Most notably, these reforms provoked opposition from powerful Lithuanian magnates like Stanisław Szczęsny Potocki, who sided with Russia in forming the Confederation of Targowica, which invited Russia to invade and restore the old order. The Polish–Russian War of 1792 and the rise of the Targowica Confederation marked the beginning of the Commonwealth’s irreversible collapse.
The Polish army, under Prince Józef Poniatowski and Tadeusz Kościuszko, fought bravely but was overwhelmed. Lithuania fell quickly due to betrayal by Duke Louis of Württemberg and poor leadership. Despite some tactical victories, such as the Battle of Zieleńce, King Stanisław August Poniatowski, under pressure from Catherine, ordered a halt to military resistance, effectively surrendering to the Confederation. This capitulation dismantled the progress of the Great Sejm and placed the Commonwealth under Russian control.
Following the Russian victory, Targowica leaders established a reactionary regime, undoing reforms and suppressing Enlightenment ideals. However, Russia and Prussia, seeing the weakened state as ripe for exploitation, negotiated the Second Partition of Poland in 1793. Prussia annexed Greater Poland, Thorn (Toruń), and Danzig (Gdańsk), while Russia absorbed large portions of Belarus and Ukraine, leaving the Commonwealth a mere shadow of its former self.
The partition left Lithuania under the de facto rule of pro-Russian magnates, such as the Kossakowski brothers, governing in the name of the Tsar. The puppet Grodno Sejm, held under Russian military supervision, legitimized the territorial cessions. Although the remaining Commonwealth was nominally independent, it functioned as a Russian protectorate. The partitions and Targowica's betrayal set the stage for future uprisings, including the Kościuszko Uprising, and ultimately led to the Third Partition (1795), erasing Poland and Lithuania from the map until the 20th century.