The Princedom of Pontecorvo was a short-lived client state of France during the Napoleonic Wars.
Napoleon took it from the Papal State after becoming King of Italy, created it as a princedom and made one of his Imperial Marshals the Prince of Pontecorvo.
The Marshal, Jean-Baptiste Bernadotte, was soon headhunted by the Swedish establishment to become Karl XIV Johan, the new King of Sweden, and gave up the princedom. His dynasty formally rules Sweden to this day.
Pontecorvo today has a population of ~13000, the same size population as the bunch of towers on top of a shopping mall where I live.