Left without an obvious next head of state, Poland took it upon itself to elect one. Thus began the First Republic of Poland which survived until the final partition of 1795. An apparent advance on one level, the Sejm cost Poland on others. In theory, the enfranchised nobility represented 10% of Poland's population, but in practice, a powerful few controlled them all. Those few solidified their power by weakening the king's, leaving the strength of the elected position dependent upon the individual that held it. If they didn't want a strong king, they didn't elect one.
The Republic also ensured that the feudal state survived, and it kept capitalism captive in the mostly German and Jewish dominated cities. Self-interested nobles failed to raise the taxes to support a standing army, and the advances achieved under the Jagiellonian dynasty quickly crumbled away. In the mid-17th century, Poland was invaded and pillaged by Sweden. The resulting war devastated the country and the people.
But for a brief time afterwards, Poland re-emerged under the reign of Jan III Sobieski, most noted for his decisive role in the Battle of Vienna. In 1683, the Ottoman Empire had advanced northward, eventually laying siege to Vienna. Considered unbeatable at the time, a Christian
Europe was given up for lost until Sobieski showed on the scene. Saving the Austrian Empire cost Sobieski on another front; the Prussians took advantage of the battle-wearied Poland, which barely resisted.
Also under the First Republic, Sejm members began to misuse one of their most serious privileges: the right of veto. Assuming that an elected body acted according to the wishes of the people, all decisions were to be unanimous. This meant that any member could veto an act of the Sejm, voiding it for good. The first time such a veto was used, in 1652, the dissenting deputy left and the session ended without final approval of any legislation. An idea with the best of intentions in practice led to the downfall and eventual loss of the country itself.
With Sejm business at the mercy of any self-interested member, the resulting fragmented power base left an opening for the land-hungry empires of Prussia-Germany, Austria-Hungary, and Russia.
Each in their own time began to chip away at the unity of Poland, buying a vote here and there, and eventually bought enough to bring governance to a standstill. By the time Poles woke up, it was a bit too late. The first partition of Poland took place in 1772 when Russia realized it would be easier to cede land to Austria and Prussia than fight for it. The shock did Poland some good, and in the next twenty years a mini-Renaissance briefly revived the country and culminated in the first Constitution in Europe in 1791.
But the neighboring powers were not satisfied with their initial land grabs, so in 1793 and 1795 they finished what they started and Poland as a political and geographic entity ceased to exist. It was not to emerge again until the end of World War I.