He signed the Treaty of Gallipoli with the Byzantine regent John VII Palaiologos in 1403.. By this treaty, he gave up certain territories along the Marmara coast to the Byzantine Empirein return for Byzantine support in interregnum. He declared himself as the sultan of the empire in Edirne, the co capital in Rumeli of the Ottoman Empire. But the Asiatic side of the empire, so called Anatolia, was under the control of his two brothers İsa Çelebi and Mehmet Çelebi. Süleyman supported İsa against Mehmet. However, Mehmet defeated İsa in several battles in 1406. Afraid of Mehmet's increasing power, Süleyman crossed the straits to reunite the empire. He captured Bursa, the Anatolian capital. But before fighting against Mehmet, he marched to the Aegean Region to intimidate the small Turkmen principalies which had been annexed by the brothers' father Beyazid I, but had broken free after the disastrous Battle of Ankara. He then captured the city of Ankara from Mehmet but did not advance further. Süleyman returned to Bursa, which gave Mehmet a chance to relax. Mehmet then made an alliance with their brother Musa Çelebi, who was also a contender for the Ottoman throne, sending Musa to the European portion of the empire via Wallachia. Because of this plot, Süleyman now had to fight in two fronts on two sides, one in Europe against Musa and one in Anatolia against Mehmet. Süleyman turned his attention to Rumelia against Musa, leaving Anatolia to Mehmet once again. Musa had the support of Wallachians and the Serbs, and Süleyman had the support of the Byzantines. However, the Serbs switched sides and joined Süleyman's forces, and Musa was defeated in the Battle of Kosmidion on 15 June 1410. However, Süleyman was not a willful prince, and to the dismay of his partisans, he began living in extravagance. Especially after the death of his able vizier Çandarlı Ali Pasha, Süleyman's indifference to state affairs caused him to lose supporters. Thus in 1411, when Musa marched to Edirne, Süleyman found almost no one at his side. He tried to escape to Byzantine territories, but on the way, he was murdered on February 17, 1411.
Aftermath
After Süleyman's death, Musa became the ruler of the Rumeli. The alliance between Mehmet and Musa soon broke and the two brothers continued to fight until Musa's defeat and death on July 5, 1413, when Mehmet became the sole ruler of the empire as Mehmet I.