Isa and Mehmed

Civil war broke out among the sons of Sultan Bayezid I upon his death in 1403. His oldest son, Süleyman, with his capital at Edirne, ruled the recently conquered Bulgaria, all of Thrace, Macedonia and northern Greece. The second son, İsa Çelebi, established himself as an independent ruler at Bursa^[11] and Mehmed formed a kingdom at Amasya.^[12] War broke out between Mehmed and İsa, and following the battles of Ermeni-beli^[13] and Ulubad (March–May 1403),^[11] Isa fled to Constantinople and Mehmed occupied Bursa.^[14] The subsequent battle at Karasi between Mehmed and Isa resulted in a victory for Mehmed and Isa fleeing to Karaman.^[13] Isa was later killed in a bath by agents of Mehmed.^[15]

Suleyman enters civil war

Meanwhile, the other surviving son of Bayezid, Musa Çelebi, who was captured at the battle of Ankara, was released by Timur into the custody of Yakub of Germiyan.^[16] Mûsa was freed, after Mehmed made a request for his brother's release. Following Isa's death, Süleyman crossed the straits with a large army.^[17] Initially, Süleyman was successful. He invaded Anatolia, capturing Bursa (March 1404)^[18] and Ankara later that year.

During the stalemate in Anatolia, which lasted from 1405–1410, Mehmed sent Musa across the Black Sea to Thrace with a small force to attack Suleyman's territories in south-eastern Europe. This maneuver soon recalled Suleyman to Thrace, where a short but sanguinary contest between him and Mûsa ensued. At first Suleyman had the advantage, winning the battle of Kosmidion in 1410, but in 1411 his army defected to Mûsa at Edirne. Suleyman was captured, given to Musa's bodyguard, Koyun Musasi, and strangled to death on 17 February 1411.^[19][20] Mûsa was now the ruler of the Ottoman dominions in Thrace.

Mehmed and Musa

Manuel II Palaiologos, the Byzantine emperor, had been the ally of Suleyman; Mûsa therefore besieged Constantinople.^[21] Manuel called on Mehmed to protect him, and Mehmed's Ottomans now garrisoned Constantinople against Musa's Ottomans of Thrace. Mehmed made several unsuccessful sallies against his brother's troops, and was obliged to re-cross the Bosporus to quell a revolt that had broken out in his own territories. Mûsa now pressed the siege of Constantinople. Mehmed returned to Thrace, and obtained the assistance of Stefan Lazarevic, the Serbian Despot.

The armies of the rival Ottoman brothers met on the plain of Chamurli (today Samokov, Bulgaria). Hassan, the Agha of the Janissaries of Mehmed, stepped out before the ranks and tried to get the troops to change sides. Mûsa rushed towards Hassan and killed him, but was himself wounded by an officer who had accompanied Hassan. Mûsa's Ottomans fought well, but the battle was won by Mehmed and his allies.^[22] Mûsa fled, was later captured and strangled.^[23] With Mûsa dead, Mehmed was the sole surviving son of the late Sultan Bayezid I and became Sultan Mehmed I.

The Interregnum was a striking example of the fratricide that would become common in Ottoman successions.