Since the union's establishment, it faced attacks from the
Byzantine Empire. They concluded an alliance with the Bulgars in the 670s after the latter crossed the
Danube River.
Theophanes writes that the Bulgars became masters of the Slavs. The Seven Tribes recognized the sovereignty of Khan
Asparuh, and together, in the spring of 681, they won a major victory over the Byzantines. In the late 7th century the Seven Tribes were assigned the defence of the newly-established Bulgar Khanate's western and northwestern border (the
Iskar River up to its mouth in the
Danube) against
Avar raids, as well as some of the passes of the
Balkan Mountains. The
Severi, whose possible participation in the union is unclear, guarded the eastern part of the mountains. The Seven Slavic Tribes, together with other Slavic and non-Slavic tribes of the Bulgarian Empire, gradually coalesced into a single
Bulgarian people group in the 9th century. The reasons for this ethnic formation included the
Christianization of Bulgaria under
Boris I and the preceding administrative reforms that deprived the various tribes of their autonomy and self-government. ==See also==