When Ottoman Turks captured
Edirne (Adrianopolis), Byzantine emperor
John V Palaiologos appealed to the West for help. Instead, he was detained as a debtor in
Venice. Andronikos (later
Andronikos IV Palaiologos), his son and regent in Constantinople (modern
İstanbul,
Turkey), refused to pay the ransom for his father, and John had to give up the island Tenedos (modern
Bozcaada, Turkey) to buy his freedom. After that event, John assigned his younger son Manuel (later
Manuel II Palaiologos) as his crown prince and accepted the suzerainty of Ottomans in 1373. Thus, when the Ottoman sultan asked for his services against some rebellions in Ottoman lands, he had to leave his capital. This absence gave Andronikos a chance to rebel.
On the Ottoman side, Savcı Bey, who was the youngest of three brothers, saw that under the shadow of his older brothers, he had almost no chance to be enthroned in the future and faced a probable death under
the traditional policy of fratricide in Ottoman succession. (This fear was not unreasonable; later when Murat I died older brother
Beyazit I immediately killed the other brother). He prepared to rebel to gain the post. While his father was occupied with suppressing the rebellions, Savcı saw his chance to revolt. Using the royal treasury under his disposal, he formed an army. The two rebellious princes, well aware of one another's interests, decided to collaborate and combined their forces
Many more examples.