When it finally became established as a modern independent nation-state, it was – despite a very brief flirtation with monarchy – in the form of a republic.
In 1742, following the Russian occupation of Finland in the Russo-Swedish War (1741–1743) and vague promises of making the country independent, the four estates gathered in Turku and decided to ask Empress Elizabeth of Russia if the then Duke Peter of Holstein-Gottorp, grand-nephew of the late king Charles XII of Sweden, could be proclaimed as the King of Finland.
The Russian Emperor wielded the powers formerly reserved for the King of Sweden as the Grand Prince of Finland, creatively applying the autocratic Swedish constitution of 1772 and 1789.
The internal unrest in the country soon descended into the Finnish Civil War, an open civil war between the Reds (i.e. revolutionary forces consisting of various socialist factions and the working class) and the Whites (i.e. pro-government forces, political conservatives and much of the middle and upper classes).
Before Frederick Charles could move to Finland, however, the collapse of the Central Powers made the idea of German-born Finnish king untenable and he declined the throne.