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The Byzantine Empire and the Sultanate of Rûm before the Crusades.
Byzantines grant him the crown of basileus (emperor) of Bulgaria and had the young emperor Constantine VII marry one of his daughters. When a revolt in Constantinople halted his dynastic project, he again invaded Thrace and conquered Adrianople.
A great imperial expedition under Leo Phocas and Romanos Lekapenos ended again with a crushing Byzantine defeat at the Battle of Acheloos (917), and the following year the Bulgarians were free to ravage northern Greece as far as Corinth. Adrianople was captured again in 923 and in 924 a Bulgarian army laid siege to Constantinople. The situation in the Balkans improved only after Simeon's death in 927. In 968, Bulgaria was overrun by the Rus' under Sviatoslav I of Kiev, but three years later, the emperor John I Tzimiskes defeated the Rus' and re-incorporated eastern Bulgaria into the Empire.
Bulgarian resistance revived under the rule of the Cometopuli dynasty, but the new emperor Basil II (reigned 976–1025) made the submission of the Bulgarians his primary goal. Basil's first expedition against Bulgaria however resulted in a humiliating defeat at the Gates of Trajan. For the next few years, the emperor would be preoccupied with internal revolts in Anatolia, while the Bulgarians expanded their realm in the Balkans. The war was to drag on for nearly twenty years. The Byzantine victories of Spercheios and Skopje decisively weakened the Bulgarian army, and in annual campaigns, Basil methodically reduced the Bulgarian strongholds. Eventually, at the Battle of Kleidion in 1014
the Bulgarians were completely defeated. The Bulgarian army was captured, and it is said that 99 out of every 100 men were blinded, with the remaining hundredth man left with one eye so as to lead his compatriots home. When Tsar Samuil saw the broken remains of his once gallant army, he died of shock. By 1018, the last Bulgarian strongholds had surrendered, and the country became part of the Byzantine empire. This victory restored the Danube frontier, which had not been held since the days of the emperor Heraclius.
Between 850 and 1100, the Empire developed a mixed relationship with a new state that emerged to the north across the Black Sea, that of the Kievan Rus'. This relationship would have long-lasting repercussions in the history of East Slavs. Byzantium quickly became the main trading and cultural partner for