The Byzantine Empire was a European empire with most of its territory outside Europe that was the direct continuation of the Roman Empire. Founded as a Christian empire, it retained a lot of strength even after the collapse of the Western Roman Empire. However, over time, Slavic invasions sapped it of quite a bit of territory. In 628, the Byzantine Emperor Heraclius converted to Islam after receiving a letter from Muhammed. Despite losing territory to Christians, the empire became more united by the religion. Fighting in the Saracen Crusade along with the Talibid Caliphate, the Empire strengthened as the fact that the Ummah defeated the might of the Christendom have glory to the Empire. However, soon afterwards, the Talibids, highly critical of Greek influence on them, declared war on the Byzantines. The Byzantines quickly won the war and the Golden Age of Islam began. Trade with India led to the introduction of Hindu numerals to the Byzantines. However, even in the Golden Age, the Byzantines were slowly collapsing. By 1100, all territory except for Anatolia and historical Greece was outside Byzantine control. However, in 1113, the Nestorian Christian Temour Empire invaded Anatolia. The wave of Turks nearly conquered Constantinople. This was a devastating blow that the Byzantines never recovered from. The Byzantine Empire, now restricted to Greece, Thrace, and Dalmatia, was devastated by the storm of Turks. However, its remaining territory was consolidated and, when the Mongol Empire invaded Anatolia, it also sacked Constantinople. The Golden Age of Islam ended and Dalmatia fell to Slavic invasions. Despite being Muslim, the Slavs were no friends of the Byzantines.