A new period of expansion, in which Sultan Bayezid's empire is restored and new territories are added, occurs under Bayezid's son Mehmed I, and Mehmed's son Murad II.

Mehmed restores the vassal system in Bulgaria and Serbia, promising that he will not undertake new European adventures.

Murad II is also compelled to devote most of the early years of his reign to internal problems, particularly to the efforts of the ghazi commanders and Balkan vassal princes in Europe, as well as the Turkmen vassals and princes in Anatolia, to retain the autonomy and—in some areas—independence that had been gained during the Ottoman Interregnum from 1402 to 1413.

Murad suppresses the Balkan resistance in 1422-23 and puts Constantinople under a new siege that ends only after the Empire provides him with huge amounts of tribute.

He then restores Ottoman rule in Anatolia and eliminates all Turkmen principalities left by Timur, with the exceptions of Karaman and Candar, which he leaves autonomous though tributary so as not to excite the renewed fears of Timur's successors in the East.

Placed on the throne by Turkish notables who had joined the Ottoman state during the first century of its existence, Murad soon begins to resent the power they have gained in return; the power of these notables is also enhanced by the great new estates they have built up in the conquered areas of Europe and Anatolia.

To counteract their power, he begins to build up the power of various non-Turkish groups in his service, particularly those composed of Christian slaves and converts to Islam, whose military arm is organized into a new infantry organization called the Janissary corps.

To strengthen this group Murad begins to distribute most of his new conquests to its members, and to add new supporters of this sort he develops the famous devshirme system, by which Christian youths are drafted from the Balkan provinces for conversion to Islam and life service to the sultan.