For hundreds of years the Byzantine army held back the various waves of Muslim and Steppe invaders because of an effective and efficient defense in depth throughout the empire. The first layer was a superior intelligence system, managed from the “Office of Barbarians” that tracked the movements of tribes on the Steppes, and the activities of the sultans. It gave sufficient warning for Byzantine diplomats to bribe a rival tribe or sultan into war against the impending invader. The next layer was the buffer states, Georgia, Armenia etc which could be reinforced or let fall as needed, but gave time for the next two layers: The first was a series of well stocked, manned and provisioned border fortresses, which could hold out for years if necessary. The next and most important were the troops of the “themes” or provinces, comprised of free peasants who served in times of crisis in exchange for land. Finally, the thematic troops were backed up by the semi-autonomous regional tagmata, or professional armies. All of which if necessary could be reinforced by the Emperor’s personal guard and the nobles levy from Constantinople.
The system was amazingly flexible and effective on the defense but cumbersome on the offense. In the early 11th century, when the threat of invasion seemed remote, the emperors wanted to recover lost land, and began reorganizing it to better suit offensive operations. Unfortunately, they undermined the critical foundation, the self sufficient thematic peasants, whom made amazing soldiers, chiefly because they, being free, were well equipped, and they had skin in the game defending their own lands. However, that came at a cost because they weren’t productive if they were deployed. The emperors gradually mobilized them less and less, but took increasing portions of their goods, not to pay for more tagmata, which potentially posed a threat to their reign, but to pay for mercenaries loyal directly to emperor (and that didn’t have to return for the harvest). Finally, in their attempt to expand the empire, they directly annexed in a bloody and destructive war, Armenia – the bulwark against the east and north east. The tenacity in which the Armenians fought the steppe invaders was turned against the Byzantines, and it would result in the virtual destruction of the country and the first Armenian diaspora. More importantly, it meant no Armenian buffer.
In the mid eleventh century, the Byzantines were victorious and modestly expanded the empire, but it did not mattter. In 1070, the Seljuk Turks took advantage of the Armenian situation and crashed through the weakened area, and overran the border fortresses devoid of thematic troops. The next year, Emperor Romanos himself led the army against the Seljuk Turks under Sultan Alp Arslan, and on 26 August, was decisively defeated at the Battle of Manzikert in what is now eastern Turkey. The hardcore of the Byzantine Army was destroyed, including all of the tagmata, the households of all the major nobles, and the famed Varangian Guard, the intensely loyal viking mercenaries whom died to a man defending the emperor. Emperor Romanos was captured, and within six years most of Eastern and Central Anatolia would fall.
The Battle of Manzikert would be known as “The Disaster” for the next 500 years. More immediately, 13 years later in 1094, the Byzantines, still reeling from the complete loss of the professional core of its army, would ask the Pope for help against the Turks, leading to the First Crusade.