The duchy emerged from a deal Napoleon struck with Tsar Alexander I and the Prussian king on a raft in the middle of the Niemen River in July 1807. Prussia, having lost the war that ended at Friedland, had to surrender most of its Polish territories. Napoleon, who needed allies on his eastern flank and had been courted relentlessly by exiled Polish patriots in Paris, used those territories to create a new state. He called it the Duchy of Warsaw and put Frederick Augustus I of Saxony on its throne in personal union with his Saxon kingdom - the same Wettin dynasty that had once provided Poland with two of its kings. In 1809 the duchy expanded south after winning the Battle of Raszyn against Austria, gaining Krakow, Lwow, and other lands lost in the partitions. By 1810 it covered roughly 155,000 square kilometers and held over four million people, 79 percent of them ethnic Poles.

The Constitution Napoleon imposed on the duchy in 1807 was, by the standards of the day, genuinely liberal. It established a bicameral Sejm with a Senate and a Chamber of Deputies. It guaranteed equality before the law for all classes, although the nobility retained outsized representation in the legislature. It declared Roman Catholicism the state religion while protecting the rights of other faiths. Most strikingly, it abolished serfdom - granting the peasantry personal freedom, even if the absence of land reform meant most peasants remained economically dependent on their former lords. But the constitution also revealed what the duchy really was. The most powerful person in Warsaw was the French ambassador. The state had no diplomatic representatives abroad. Frederick Augustus's actual rule was subordinated entirely to French raison d'etat, and the duchy existed primarily to provide France with troops and resources for Napoleon's wars.

Few small states have ever militarized so heavily as the Duchy of Warsaw. From a population of just over four million, the duchy raised an army that grew from 30,000 regulars in 1807 to nearly 120,000 men by Napoleon's 1812 invasion of Russia - a force comparable in size to the entire army Napoleon brought to Austerlitz from a country six times larger. Prince Jozef Poniatowski, the duchy's war minister, was eventually elevated to Marshal of France, the only foreigner so honored during the Empire. Polish lancers and cavalry served on every major Napoleonic battlefield from Spain to Russia. The hopes pinned on Napoleon were enormous - many Poles believed he would resurrect not just the duchy but the full Polish-Lithuanian Commonwealth, perhaps with Lithuania, Volhynia, and Podolia restored. On June 28, 1812, the Sejm formed the General Confederation of the Kingdom of Poland to make exactly that demand. Napoleon, weighing a possible peace with Russia, declined to recognize it.

The catastrophe of the Russian campaign destroyed both the Grande Armee and the duchy's chances. Of the Polish troops who marched east, few returned. By January 1813 Russian armies had occupied most of the duchy's territory, and the Vistula fortresses held out only briefly. At the Congress of Vienna in 1815 the great powers debated what to do with what was left. Russia took the largest share, transforming the central territory into the so-called Congress Kingdom of Poland in personal union with the tsar - a Russian puppet state that lost even its nominal autonomy after the failed November Uprising of 1830-31. Prussia carved out the Grand Duchy of Posen in the west. Krakow became a tiny Free City under three-power protection until Austria swallowed it in 1846 after the failed Krakow Uprising. The duchy as Napoleon designed it was gone. But for a generation of Poles who had served in its army and lived under its laws, it had proven that Poland could exist again. That memory survived 123 more years of partition until, in 1918, it could finally come true.