Background

By 1790, on the political front, the Commonwealth had deteriorated into such a helpless condition that it was successfully forced into an unnatural and ultimately deadly alliance with its enemy, Prussia. The Polish-Prussian Pact of 1790 was signed, giving false hope that the Commonwealth might have at last found an ally that would shield it while it reformed itself. The May Constitution of 1791 enfranchised the bourgeoisie, established the separation of the three branches of government, and eliminated the abuses of Repnin Sejm. Those reforms prompted aggressive actions on the part of its neighbours, wary of the potential renaissance of the Commonwealth. Once again Poland dared to reform and improve itself without Russia's permission, and once again the Empress Catherine was angered; arguing that Poland had fallen prey to the radical Jacobinism then at high tide in France, Russian forces invaded the Commonwealth in 1792.

In the War in Defense of the Constitution, the army of the Russian Empire, invited by the pro-Russian, conservative, Polish magnates' alliance, the Confederation of Targowica, fought against the Polish forces supporting the Constitution. The conservative nobility (szlachta) believed that the Russians would help them restore their Golden Liberty. Abandoned by their Prussian allies, the badly outnumbered Polish pro-Constitution forces fought under Prince Józef Poniatowski a defensive war with some measure of success, but were ultimately ordered to abandon their efforts by their supreme commander, King Stanisław August Poniatowski. The King decided to join the Targowica Confederation, as demanded by the Russians.

Russia invaded Poland to ensure the defeat of the Polish reforms, with no overt goal of another partition (it viewed Poland as its protectorate, and saw little need in giving up chunks of Poland to other countries). Frederick William II of Prussia, however, saw those events as an opportunity to strengthen his country. Frederick demanded from Catherine that for his country's abandoning Poland as a close ally, and for Prussian participation in the War of the First Coalition against revolutionary France (a participation that Russia encouraged, and in which Prussia recently suffered a major defeat at the Battle of Valmy), Prussia should be compensated – preferably with parts of the Polish territory. Russia soon decided to accept the Prussian offer.

Partition

On 23 January 1793 Prussia signed a treaty with Russia, agreeing that Polish reforms would be revoked and both countries would receive chunks of Commonwealth territory. Russian and Prussian military took control of the territories they claimed soon afterward, with Russian troops already present, and Prussian troops meeting only nominal resistance. In 1793, deputies to the Grodno Sejm, last Sejm of the Commonwealth, in the presence of the Russian forces, agreed to the Russian and Prussian territorial demands. The Grodno Sejm became infamous not only as the last sejm of the Commonwealth, but because its deputies were bribed and coerced by the Russian soldiers, as Russia and Prussia wanted official, legal approval from Poland of their new demands.

Russia took 250,000 square kilometres (97,000 sq mi), while Prussia took 58,000 square kilometres (22,000 sq mi). The Commonwealth lost about 307 000 km², being reduced to 215 000 km².

Russia received the Kiev Voivodeship, Bracław Voivodeship, Podole Voivodeship and Minsk Voivodeship, and parts of the Vilnius Voivodeship, Nowogródek Voivodeship, Brest Litovsk Voivodeship and the Volhynian Voivodeship. This was accepted by the Grodno Sejm on 22 July. Russia reorganized its newly acquired territories into Minsk Viceroyalty and Izyaslav Viceroyalty (which in 1795 was split into Podolian and Volhynian Viceroyalties).

Prussia received the cities of Gdańsk (Danzig) and Toruń (Thorn), and Gniezno Voivodeship, Poznań Voivodeship, Sieradz Voivodeship, Kalisz Voivodeship, Płock Voivodeship, Brześć Kujawski Voivodeship, Inowrocław Voivodeship, Dobrzyń Land, and parts of the Kraków Voivodeship, Rawa Voivodeship and Masovian Voivodeship. This was accepted by the Grodno Sejm on 23 September or 25 September (sources vary). Prussia organized its newly acquired territories into South Prussia.

The Commonwealth lost about 5 million people; only about 4 million people remained in the Polish-Lithuanian lands.

What was left of the Commonwealth was merely a small buffer state with a puppet king, and Russian garrisons keeping an eye on the reduced Polish army.

Aftermath

Targowica confederates, who did not expect another partition, and the king, Stanisław August Poniatowski, who joined them near the end, both lost much prestige and support. The reformers, on the other hand, were attracting increasing support. In March 1794 the Kościuszko Uprising begun. The defeat of the Uprising in November that year resulted in the final Third Partition of Poland, ending the existence of the Commonwealth.