Rumeli in the period of dynastic instability : why were the Ottoman Balkans so important for the dynasty in the first half of the 15th century?

As Peter Mentzel states, the Balkans (Rumeli) were not only a borderland but also the core province of the early Ottoman state. The Rumelian military aristocracy played one of the most important roles in the internal policy. It constituted an important factor, which was powerful enough to create the Ottoman policy. That is why Murad I forbade the Ottoman princes to lead the akıncı warriors in order to avoid the risk of a dynastic war. He also started devshirme among Christian families in the Balkans so as to build trustworthy groups of servants for the dynasty. Obviously, the province gained importance in the difficult times after the defeat at Ankara (1402). During the civil war (1402-1413, fetret devri) Rumeli was governed by one of the brothers who claimed power over the whole Ottoman territory. The deciding struggles between the sons of Bayezid I took place in the Balkans and their result depended mainly on the attitudes shown by the Rumelian warriors and their frontier lords. The rulers who lost the support of the Rumelian military class quickly lost the throne of Rumeli as well. It happened in the cases of Emir Süleyman, prince Musa, and Düzme Mustafa.