How did Sardinia become part of the kingdom ruled by the house of Savoy?

Sardinia became part of the kingdom ruled by the house of Savoy as a result of international diplomacy following the War of the Spanish Succession.

After the island was briefly recaptured by Spain in 1717, the Treaty of London was signed in 1718 among the great powers. Under this agreement, Victor Amadeus II, the duke of Savoy and sovereign of Piedmont, was forced to yield Sicily to the Austrian Habsburgs. In exchange, he was granted the title of king of Sardinia.12

Victor Amadeus II formally took possession of the island on August 24, 1720. Although the rulers were thereafter known as kings of Sardinia, the seat of their power and wealth remained in Piedmont until the eventual unification of Italy in 1861.12