Despite a number of military setbacks, the reign of Boris I was marked with significant events that shaped Bulgarian and European history.

A skillful diplomat, Boris I successfully exploited the conflict between the Patriarchate of Constantinople and the Papacy to secure an autocephalous Bulgarian Church, thus dealing with the nobility's concerns about Byzantine interference in Bulgaria's internal affairs.

The only direct evidence of Boris's title are his seals and the inscription found near the town of Ballsh, modern Albania, and at Varna.

When Charlemagne was proclaimed Emperor of the Holy Roman Empire by Leo III, the Pope was effectively nullifying the legitimacy of Irene.

[13][14] At the time of his accession he threatened the Byzantines with an invasion, but his armies did not attack,[15] and he received a small area in Strandzha to the southeast.

Both peoples had coexisted peacefully up to that time, suggesting that the Croats were paid by Louis to attack Bulgaria and distract Boris' attention from his alliance with Great Moravia.

[21] As a result of the military actions in 855, the peace between Bulgaria and Eastern Francia was restored, and Rastislav was forced to fight against Louis alone.

In 853 or 854, the Bulgarian army led by Vladimir-Rasate, the son of Boris I, invaded Serbia, with the aim of replacing the Byzantine overlordship over the Serbs.

[30] A more mythological version is the one in which Boris is astonished and frightened by an icon of Judgement day and thus decides to adopt Christianity.

In order to both extend his control over the Slavic world and gain an ally against one of the most powerful foes of the Bulgars, the Byzantine Empire, Boris sought to establish an alliance with Louis the German against Ratislav of Moravia.

[32] Through this alliance, Louis promised to supply Boris with missionaries, which would have effectively brought the Bulgars under the Roman Church.

[33] However, late in 863, the Byzantine Empire under Emperor Michael III declared war on Boris and the Bulgars during a period of famine and natural disasters.

[34] At the beginning of 864, Boris was secretly baptized at Pliska by an embassy of Byzantine clergymen, together with his family and select members of the Bulgarian nobility.

The arrival of Methodius and his followers introduced the Cyrillic alphabet, freeing the Bulgarians from dependence on Greek as a written and liturgical language.

[36] At the same time Boris sought further instruction on how to lead a Christian lifestyle and society and how to set up an autocephalous church from the Byzantine Patriarch Photios.

The pope temporarily glossed over the controversial question of the autocephalous status desired by Boris for his church and sent a large group of missionaries to continue the conversion of Bulgaria in accordance with the western rite.

To deliver his response to Boris’ questions, Pope Nicholas I sent two bishops to Bulgaria: Paul of Populonia and Formosus of Porto.

The Pope expected that these priests would execute their episcopal responsibilities to address Boris’ concerns, but did not intend for them to be elevated to the positions that they assumed in the Bulgar hierarchy.

This determined the future of the Bulgarian Orthodox Church, which was granted the status of an autocephalous archbishopric by the Patriarchate of Constantinople and an archbishop of its own.

[41] In 885, Boris was presented with a new opportunity to establish a native clergy when Slavic-speaking disciples of St. Cyril and St. Methodius were forced to flee from Moravia after a German-inspired reaction to the death of the apostle.

Boris happily greeted two of these disciples, Clement of Ohrid and Naum of Preslav, who were of noble Bulgarian Slavic origin.

In Bulgaria, Clement of Ohrid and Naum of Preslav created (or rather compiled) the new Bulgarian script, later called Cyrillic that was declared the official alphabet in 893.

Boris gathered the Council of Preslav placing his third son, Tsar Simeon I of Bulgaria, on the throne, threatening him with the same fate if he too apostatized.

Boris returned to his monastery, emerging once again in c. 895 to help Simeon fight the Magyars, who had invaded Bulgaria in alliance with the Byzantines.