Quote:

“Instead of establishing an empire of their own, however, the Serbian nation was destined to help build the Turkish one. The first agreement concluded between Bayezid I and Stefan Lazarević after the Battle of Kosovo is described by Doukas (Book IV), who writes of Bayezid:

“Having taken hostages and concluded a treaty, he secured the promise that Stephen, the son of Lazar, would accompany him with his army wherever Bayezid campaigned.”

More important still for the Ottomans was the role of the Serbian people after they had completely lost their independence and their young men entered the Sultan’s service in both war and government. The Serbs furnished the Ottoman Empire with a considerable number of its most distinguished grand viziers. This was already the case under Bayezid II.”

Source

Serbien und die Türkei im neunzehnten Jahrhundert Volume 2. Ranke. 1879. Duncker und Humblot. Basel, University Library. p. 524