state, i.e. the language of the administration and military rule of the Avar Khaganate in Eastern Europe. In 626, the Slavs, Persians and Avars jointly attacked the Byzantine Empire and participated in the Siege of Constantinople. In that campaign, the Slavs fought under Avar officers. There is an ongoing controversy over whether the Slavs might then have been a military caste under the khaganate rather than an ethnicity. Their language—at first possibly only one local speech—once koinéized, became a lingua franca of the Avar state. This might explain how Proto-Slavic spread to the Balkans and the areas of the Danube basin, and would also explain why the Avars were assimilated so fast, leaving practically no linguistic traces, and that Proto-Slavic was so unusually uniform. However, such a theory fails to explain how Slavic spread to Eastern Europe, an area that had no historical links with the Avar Khanate. That said, the Avar state was later replaced by the definitively Slavic state of Great Moravia, which could have played the same role. It is also likely that the expansion of Slavic occurred with the assimilation of Iranic-speaking groups such as the Sarmatians, who quickly adopted Proto-Slavic due to speaking related Indo-European satem languages, in much the same way Latin expanded by assimilating the Celtic speakers in continental Western Europe and the Dacians. That sudden expansion of Proto-Slavic erased most of the idioms of the Balto-Slavic dialect continuum, which left us today with only two groups, Baltic and Slavic (or East Baltic, West Baltic, and Slavic in the minority view). This secession of the Balto-Slavic dialect ancestral to Proto-Slavic is estimated on archaeological and glottochronological criteria to have occurred sometime in the period 1500–1000 BCE. Hydronymic evidence suggests that Baltic languages were once spoken in much wider territory than the one they cover today, all the way to Moscow, and were later replaced by Slavic. Shared features of the Balto-Slavic languages The degree of relationship of the Baltic and Slavic languages is indicated by a series of common innovations not shared with other Indo-European languages, and by the relative chronology of these innovations which can be established. The Baltic and Slavic languages also share some inherited words. These are either not found at all in other Indo-European languages (except when borrowed) or are inherited from Proto-Indo-European but have undergone identical changes in meaning when compared to other Indo-European languages. This indicates that