Short answers, pulled from the story.
Józef Poniatowski was a Polish military officer and politician who served in the French Revolutionary and Napoleonic Wars, living from the 7th of May 1763 to the 19th of October 1813. He was a nephew of King Stanisław II August of Poland and later became a Marshal of the French Empire.
Józef Poniatowski died on the 19th of October 1813 during the Battle of Leipzig. He was badly wounded while trying to cover the French army's retreat and drowned in the White Elster river after French troops blew up the bridge he was trying to cross.
Józef Poniatowski was a staunch ally of Napoleon and served as minister of war and commander in chief of the army of the Duchy of Warsaw, the French client state that gave Poland renewed hope of recovering lost territory. He remained loyal even after Tsar Alexander I offered him amnesty, and he led Polish forces in the 1812 invasion of Russia and the 1813 German campaign.
Józef Poniatowski supported the 3rd of May Constitution of 1791 as a member of the Friends of the Constitution Association, and troops under his command surrounded the Royal Castle in Warsaw during the final proceedings. He stood in the room himself with a group of soldiers as the document was adopted.
Józef Poniatowski is buried in the cathedral on Kraków's Wawel Hill, beside Tadeusz Kościuszko and John III Sobieski. His remains were transported to Poland in 1817, four years after his death at Leipzig.
Józef Poniatowski led Polish forces to victory in the Austro-Polish War of 1809, fighting the Austrians to a standstill at the Battle of Raszyn on the 19th of April before advancing into Galicia. His army took Lublin, Sandomierz, Zamość, and Lwów, and the war ended in a Polish victory that let the Duchy of Warsaw recover some partitioned land.