St. John's Archcathedral (Warsaw) in the context of "Constitution of 3 May 1791"

The Constitution of 3 May 1791, titled the Government Act, was a written constitution for the Polish–Lithuanian Commonwealth that was adopted by the Great Sejm that met between 1788 and 1792. The Commonwealth was a dual monarchy comprising the Crown of the Kingdom of Poland and the Grand Duchy of Lithuania; the new constitution was intended to address political questions following a period of political agitation and gradual reform that began with the Convocation Sejm of 1764 and the election that year of the Commonwealth's last monarch, Stanisław August Poniatowski. It was the first codified, modern constitution (possessing checks and balances and a tripartite separation of powers) in Europe and the second in the world, after that of the United States.

The Constitution sought to implement a more effective constitutional monarchy, introduced political equality between townspeople and nobility, and placed the peasants under the government's protection, mitigating the worst abuses of serfdom. It banned pernicious parliamentary institutions such as the liberum veto, which had put the Sejm at the mercy of any single deputy, who could veto and thus undo all the legislation adopted by that Sejm. The Commonwealth's neighbours reacted with hostility to the adoption of the Constitution. King Frederick William II of Prussia broke the Prussian alliance with the Commonwealth, joining with Imperial Russia under Catherine the Great and the anti-reform Targowica Confederation of Polish-Lithuanian magnates, to defeat the Commonwealth in the Polish–Russian War of 1792.