In 1394, Bayezid laid siege(long blockade) to Constantinople, the capital of the Byzantine Empire. Anadoluhisarı fortress was built between 1393 and 1394 as part of preparations for the second Ottoman siege of Constantinople, which took place in 1395. Already in 1391, the rapid Ottoman conquests in the Balkans had cut off the city from its hinterland. After constructing the fortress of Anadoluhisarı to control the Bosporus strait, from 1394 on, Bayezid tried to starve the city into submission by blockading it both by land and, less effectively, by sea. Lack of a fleet or necessary artillery to demolish those impressive walls made this an abortive siege. These lessons would later help the later Ottoman emperors. On the urgings of the Byzantine emperor Manuel II Palaeologus, a new crusade was organized to defeat him.
In 1396 Hungarian King Sigismund finally pulled together a crusade against the Ottomans. The crusader army was composed primarily of Hungarian and French knights, but included some Wallachian troops. Though nominally led by Sigismund, it lacked command cohesion. The crusaders crossed the Danube, marched through Vidin, and arrived at Nikopol, where they met the Turks. The headstrong French knights refused to follow Sigismund's battle plans, resulting in their crushing defeat. Because Sratsimir had permitted the crusaders to pass through Vidin, Bayezid invaded his lands, took him prisoner, and annexed his territories. With Vidin's fall, Bulgaria ceased to exist, becoming the first major Balkan Christian state to disappear completely by direct Ottoman conquest.
The Battle of Ankara or Angora was fought on 20 July 1402 at the Çubuk plain near Ankara, between the forces of the Ottoman Sultan Bayezid I and the Emir of the Timurid Empire, Timur. The battle was a major victory for Timur. After the battle, Timur moved through western Anatolia to the Aegean coast, where he besieged and took the city of Smyrna, a stronghold of the Christian Knights Hospitalers. The battle was catastrophic for the Ottoman state, fracturing what remained and bringing almost total collapse of the empire. Mongols roamed free in Anatolia and the political power of the sultan was broken. This resulted in a civil war among Bayezid's sons known as the Ottoman Interregnum.