Jean-Baptiste Bernadotte was the son of a French lawyer, born in 1763 in the Gascon town of Pau in the foothills of the Pyrenees. He joined the army at the bottom as a humble poilu at the age of 17 and was a full sergeant in the peacetime army by the age of 25. Competent soldiers untainted by blue blood were a desirable commodity in the French revolutionary wars and four years later he was a Colonel, and two years after that became General Bernadotte. His career in those turbulent times was full of adventures and by no means uniformly successful. But he was close to Napoleon (he married the Emperor's Ex Désirée Clary about whom the film was made) but not his 'creature'. Bernadotte had a job to do, he did it as well as he could and by all accounts he was more efficient than some of his contemporaries. Perhaps most notably he was kind.
During the campaign in Northern Germany, he treated a group of Swedish POWs with kindness while he was Governor of Hannover in 1805. He fought at the huge battles of Austerlitz (1805) but not at Jena and Auerstädt (1807) despite being close enough to weigh in. Napoleon chid him to his face and behind his back for failing to be there at the Battle of J&A. He was dramatically stripped of his command on the battlefield at Wagram (1809) for some perceived failure but as soon as he returned to Paris he was sent to oppose the British at their disastrous invasion of the Low Countries at Walcheren.
Then out of the blue he was offered a very nice position in the North. Largely because of his considerate treatment of those Swedish prisoners five year before, he was offered and accepted the post of Crown Prince and heir-presumptive to the throne of Sweden. It is also true that the powers-that-be in Sweden also wanted a competent soldier to take them through the turbulent years of war which seemed set to last forever. The old king died and Bernadotte ascended the throne on this day 5th Feb in 1818 as Carl XIV Johan. Not bad for un soldat de la France in the Régiment de Royal-Marine! One of his descendants was Folke Bernadotte whom I have mentioned before.