Byzantium's Balkan frontier advanced and retreated in the period 900–1204 in such a way that, ostensibly, it supports the picture of imperial ‘Apogee and Disintegration’ painted by George Ostrogorsky. In the early tenth century the empire experienced a period of consolidation in the northern Balkans, striking a compromise with an independent Bulgarian realm. Then began a period of expansion, punctuated by warfare but characterized by diplomacy, as regions were conquered and local power structures were absorbed and utilized. A rapid deterioration in Byzantine fortunes at the end of the eleventh century signalled that tensions within the Balkan lands, as elsewhere, were increasing. At the same time forces beyond the frontier began to encroach upon the empire from the north, and, more significantly, from the west. This put an end to further substantial territorial expansion, which, in any case would have extended the empire beyond its ‘natural’ frontier at the Danube. Therefore, the empire entered a second period of consolidation, with piecemeal expansion to the east under the first Comnenian emperors. The reign of Manuel Comnenus signalled a return to limited expansion in the west, directed against the perceived threat of western, and particularly German, encroachment. Finally came a rapid contraction of the frontier, akin to the crisis of the later eleventh century, but from which the empire did not recover.

This story of advancing and retreating frontiers takes no account of the nature of political authority in the northern Balkans.