1793
By the 1790s the First Polish Republic had deteriorated into such a helpless condition that it was successfully forced into an unnatural and ultimately deadly alliance with its enemy, Prussia. The alliance was cemented with the Polish-Prussian Pact of 1790. The conditions of the Pact were such that the succeeding and final two partitions of Poland were inevitable. 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 Poland's neighbours, wary of a potential renaissance of the Commonwealth. In the second partition, Russia and Prussia took so much territory that only one-third of the 1772 population remained in Poland.
Read more about this topic: Territorial Evolution Of Poland