Decline of the Byzantine Army AD 1071-1203

The great turning point for the Byzantine army was the battle of Manzikert in AD 1071 at which the main body of the army under command of emperor Romanus IV Diogenes was shattered by the Seljuk Turks under their Sultan Alp Arslan.

The disaster of Manzikert was followed by a mass invasion of Asia Minor (Turkey) by the Turks and a time of civil wars within the remaining Byzantine realm.

In this chaos the formidable old Byzantine army practically disappeared. Not only had Constantinople lost its army at Manzikert but with the invasion of Asia Minor it had lost its traditional recruiting grounds where to find the soldiers with whom to replace the lost regiments.

In AD 1078 emperor Michael VII Ducas collected the remaining soldiers from the former provinces of Asia Minor into a new body of cavalry - the so-called 'Immortals'. And even though he supplemented them with new recruits they numbered only ten thousand. They were the survivors of what had once been 21 themes, a force most likely well above 80'000 men.

In the face of such devastation Constantinople turned to recruiting of foreign mercenaries to help protect itself. Franks, Lombards, Russians, Patzinaks and Seljuk Turks were taken into service in the defence of what little territory remained Byzantine. Most favoured were the westerners as they were found less likely to rebel and because the sheer bravery the Frankish and Lombard warriors displayed in battle.
Though naturally the eastern horse-archers were still sought to provide their skill in ranged combat to the fierce charge of the western heavy cavalry.
Though if the troops were now largely foreign, the old tactics, the sophisticated Byzantine art of war survived in its commanders.

Even when parts of Asia Minor (Turkey) were reconquered, the military organisation of the 'themes' was not restored. Asia Minor had been so utterly devastated by the Turks, that the old recruiting grounds of the empire were barren ruins. And so the Byzantine army remained an improvised mix of various mercenary forces.

Under the emperors Alexius, John II and Manuel the Byzantine military though still managed to function quite well, despite these shortcomings. But with the death of Manuel Comnenus (AD 1180) the time of Byzantine military power faded away.
The next emperors possessed neither their predecessors' strength of leadership nor did they find the means by which to raise the money necessary to maintain an effective army.

Unpaid mercenaries make for a bad army. And so, when the Frankish knights forced their way into the city of Constantinople (AD 1203), most of the garrison - but for the Varangian Guard - refused to fight.