The term Kingdom of Sardinia denotes the Savoyard state from 1720 to 1861. From 1720 to 1847, only the island of Sardinia proper was part of the Kingdom of Sardinia, while the other mainland possessions (principally the Duchy of Savoy, Principality of Piedmont, County of Nice, Duchy of Genoa, and others) were held by the House of Savoy in their own right, hence forming a composite monarchy and a personal union, which was formally referred to as the "States of His Majesty the King of Sardinia". This situation was changed by the Perfect Fusion act of 1847, which created a unitary kingdom. Regardless, historians often use "Sardinia" as a synecdoche to designate the whole Savoyard state from 1720. Due to the fact that Piedmont was the seat of power and prominent part of the entity, the state is also referred to as Sardinia–Piedmont or Piedmont–Sardinia, and sometimes erroneously as the Kingdom of Piedmont.
Before becoming a possession of the House of Savoy, the medieval Kingdom of Sardinia had been part of the Crown of Aragon and then of the burgeoning Spanish Empire. With the Treaty of The Hague (1720), the island of Sardinia and its title of kingdom were ceded by the Habsburg and Bourbon claimants to the Spanish throne to the Duke of Savoy, Victor Amadeus II. The Savoyards united it with their historical possessions on the Italian peninsula, and the kingdom came to be progressively identified with the peninsular states, which included, besides Savoy and Aosta, dynastic possessions like the Principality of Piedmont and the County of Nice, over both of which the Savoyards had been exercising their control since the 13th century and 1388, respectively.
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