Khan Presian was inherited from his son khan Boris (852-889), one of the greatest Bulgarian rulers. He did not gain any war during his long reign but made far-sighted steps, which predetermined Bulgaria's historical fate.

In 853 khan Boris got entangled in the war against a coalition formed by the West Germanic kingdom and Croatia. In 862, this time in alliance with the West Germanic kingdom, Bulgaria waged war on Great Moravia and Byzantium. The wars were abortive for Bulgaria but they did not change the territorial status quo of the state. However the unsuccessful wars showed that enormous state was not built on a sound  foundation. The different languages and religions of the numerous tribes and peoples hindered the state's consolidation. Each religious community executed your own unwritten law and it was impossible to be enforced an united legal system for whole country. On the other hand the contacts with the European Christian countries convinced khan Boris that despite its military might Bulgaria held an inequitable position on the international stage as a result of the officially declared pagan nature of the Bulgarian state.

That is why Khan Boris and the supreme leaders of the Bulgarian aristocracy decided to adopt the Christian faith as the one and only official religion of the state. Contacts were established first with the German king Ludowig I. The peace treaty with Germanic kingdom included a clause about the intention Bulgaria to accept the new religion from the Roman Catholic church.

At this time the rivalry between West and East church was at the height.  The Orthodox Byzantium could not allow the appearance of a powerful Catholic state right at the threshold of Constantinople. Learning Bulgaria's intention the Eastern Roman emperor declared immediately war on Bulgaria. But in fact no one wanted to be at war. Bulgaria was overtook by prolonged droughts and a heavy earthquake. Byzantium despite all was afraid of his strong neighbour. That is why khan Boris consented to assume the Christianity from Constantinople and the emperor Michael III, who felt happy of the quick success, ceded to Bulgaria the region Zagore in Thrace.

In 864 special Constantinople's delegates converted khan Boris and his court to Christianity. The khan assumed the Slavonic title kniaz and the name of his godfather - the Roman emperor Michael. Kniaz Boris I Michael proclaimed Christianity as the official state religion and the conversion of all none-Christians was started.

The enforcement of Christianity did not go without perturbations. Part of the aristocracy in the external areas rose in a rebellion but it was suppressed quickly. Kniaz Boris punished cruelly the insurgents - 52 nobles and their families were sentenced to death.

But the conversion of the Bulgarian people to the Christian faith was only the beginning of a fateful change. The great life-work of kniaz Boris I Michael had not finished yet.