Byzantine Empire
867–1081

Basil I (r. 867–886) continued Michael's policies. His armies campaigned with mixed results in Italy but defeated the Paulicians of Tephrike. His successor Leo VI (r. 886–912) compiled and propagated a huge number of written works. These included the Basilika, a Greek translation of Justinian I's law-code which included over 100 new laws of Leo's devising; the Tactica, a military treatise; and the Book of the Eparch, which codified Constantinople's trading regulations. In non-literary contexts Leo was less successful: the empire lost in Sicily and against the Bulgarians, while he provoked theological scandal by marrying four times in an attempt to father a legitimate heir.

The early reign of Leo's young heir, Constantine VII, was tumultuous, as his mother Zoe, the patriarch Nicholas, the powerful Simeon I of Bulgaria, and other influential figures jockeyed for power. In 920, the admiral Romanos I used his fleet to secure power, crowning himself and demoting Constantine to the position of junior co-emperor.

Between 1021 and 1022, following years of tensions, Basil II led a series of victorious campaigns against the Kingdom of Georgia, resulting in the annexation of several Georgian provinces to the empire. Basil's successors also annexed Bagratid Armenia in 1045. Importantly, both Georgia and Armenia were significantly weakened by the Byzantine administration's policy of heavy taxation and abolishing of the levy. The weakening of Georgia and Armenia played a significant role in the Byzantine defeat at Manzikert in 1071.

Basil II is considered among the most capable Byzantine emperors and his reign as the apex of the empire in the Middle Ages. By 1025, the date of Basil II's death, the Byzantine Empire stretched from Armenia in the east to Calabria in southern Italy in the west. Many successes had been achieved, ranging from the conquest of Bulgaria to the annexation of parts of Georgia and Armenia, and the reconquests of Crete, Cyprus, and the important city of Antioch. These were not temporary tactical gains but long-term reconquests.

The Byzantine Empire then fell into a period of difficulties, caused to a large extent by the undermining of the theme system and the neglect of the military. Nikephoros II, John Tzimiskes, and Basil II shifted the emphasis of the military divisions (τάγματα, tagmata) from a reactive, defence-oriented citizen army into an army of professional career soldiers, increasingly dependent on foreign mercenaries. Mercenaries were expensive, however, and as the threat of invasion receded in the 10th century, so did the need for maintaining large garrisons and expensive fortifications. Basil II left a burgeoning treasury upon his death, but he neglected to plan for his succession. None of his immediate successors had any particular military or political talent, and the imperial administration increasingly fell into the hands of the civil service. Incompetent efforts to revive the Byzantine economy resulted in severe inflation and a debased gold currency. The army was seen as both an unnecessary expense and a political threat. Several standing local units were demobilised, further augmenting the army's dependence on mercenaries, who could be retained and dismissed on an as-needed basis.

At the same time, Byzantium was faced with new enemies. Its provinces in southern Italy were threatened by the Normans who arrived in Italy at the beginning of the 11th century. During a period of strife between Constantinople and Rome culminating in the East-West Schism of 1054, the Normans advanced gradually into Byzantine Italy. Reggio, the capital of the tagma of Calabria, was captured in 1060 by Robert Guiscard, followed by Otranto in 1068. Bari, the main Byzantine stronghold in Apulia, was besieged in August 1068 and fell in April 1071.

About 1053, Constantine IX disbanded what the historian John Skylitzes calls the "Iberian Army", which consisted of 50,000 men, and it was turned into a contemporary Drungary of the Watch. Two other knowledgeable contemporaries, the former officials Michael Attaleiates and Kekaumenos, agree with Skylitzes that by demobilising these soldiers, Constantine did catastrophic harm to the empire's eastern defences. The emergency lent weight to the military aristocracy in Anatolia, who in 1068 secured the election of one of their own, Romanos Diogenes, as emperor. In the summer of 1071, Romanos undertook a massive eastern campaign to draw the Seljuks into a general engagement with the Byzantine army. At the Battle of Manzikert, Romanos suffered a surprise defeat against Sultan Alp Arslan and was captured. Alp Arslan treated him with respect and imposed no harsh terms on the Byzantines. In Constantinople a coup put in power Michael Doukas, who soon faced the opposition of Nikephoros Bryennios and Nikephoros III Botaneiates. By 1081, the Seljuks had expanded their rule over virtually the entire Anatolian plateau from Armenia in the east to Bithynia in the west, and had established their capital at Nicaea, just 90 kilometres (56 miles) from Constantinople.
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