Stjepan Vukčić Kosača in the context of "Ottoman conquest of Bosnia and Herzegovina"

Alfonso the Magnanimous (Alfons el Magnànim in Catalan) (1396 – 27 June 1458) was King of Aragon and King of Sicily (as Alfons V) and the ruler of the Crown of Aragon from 1416 and King of Naples (as Alfons I) from 1442 until his death. He was involved with struggles to the throne of the Kingdom of Naples with Louis III of Anjou, Joanna II of Naples and their supporters, but ultimately failed and lost Naples in 1424. He recaptured it in 1442 and was crowned king of Naples. He had good relations with his vassal, Stjepan Kosača, and his ally, Skanderbeg, providing assistance in their struggles in the Balkans. He led diplomatic contacts with the Ethiopian Empire and was a prominent political figure of the early Renaissance, being a supporter of literature as well as commissioning several constructions for the Castel Nuovo.
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The House of Kosača (Serbian Cyrillic: Косача, pl. Kosače / Косаче), somewhere Kosačić (Serbian Cyrillic: Косачић, pl. Kosačići / Косачићи), was a Bosnian medieval noble family which ruled over parts of modern-day Bosnia and Herzegovina, Croatia, Montenegro, and Serbia between the 14th century and the 15th century. The land they controlled was known as Humska zemlja (Hum, for short), roughly corresponding to modern region of Herzegovina, which itself was derived from the title "Herzog", which Stjepan Vukčić Kosača adopted in 1448., with latin title "Dux Sancti Sabbae". Besides Hum, they ruled parts of Dalmatia and Rascia. They were vassals to several states, including the Kingdom of Bosnia and Ottoman Empire. Historians think the Kosača family is part of the Kőszegi family (House of Herceg), but there is a lack of evidence for this claim.

The religious confession of the Kosača family is uncertain. They were in contact with the Eastern Orthodox Church, the Church of Bosnia, the Catholic Church and Islam. During the fall of the Bosnian Kingdom, the "Kosače" split into three branches: Venetian, Dalmatian and Ottoman. From then onward, these branches became accepting of the Catholic faith, in the first two cases, and of Islam in the third.

The Duchy of Saint Sava (Serbo-Croatian: Vojvodstvo Svetog Save, Војводство Светог Саве) was a late medieval polity in southeastern Europe, that existed from 1448 up to 1482, when it was absorbed by the Ottoman Empire. It was founded and controlled by the Kosača noble family, whose rulers held the title Duke of Saint Sava (Serbo-Croatian: Herceg od Svetog Save, Херцег од Светог Саве). Their domains included southern parts of modern-day Bosnia and Herzegovina, extending to southern parts of modern-day coastal Croatia, northwestern Montenegro and southwestern Serbia. Its founder, Stjepan Vukčić Kosača (duke since 1448), titled himself Herceg of Saint Sava, a title which would later give rise to the new name to the region: Herzegovina, and will be also used by the Ottomans as Hersek Sancağı (Sanjak of the Herzeg), designating the Sanjak of Herzegovina.

In various sources, and historiographical traditions, the name of the duchy is recorded or used in several forms, varying mostly in the way the name of Saint Sava is spelled in different languages: Latin: Ducatus Sancti Sabbae, Italian: Ducato di Santo Sabba, German: Herzogtum des hl. Sava. In English historiography, forms such as the "Duchy of Saint Sava" or "Dukedom of Saint Sava" were used already since the 1700s and further on.