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Army

In the 5th century the East was fielding five armies of ~20,000 each in two army branches: stationary frontier units (limitanei) and mobile forces (comitatenses). In the 6th century, Anthony Kaldellis claims the fiscally stretched Empire could only handle one major enemy at a time. The Islamic conquests between 634 and 642 led to significant changes, transforming the 4th-7th centuries field forces into provincialised militia-like units with a core of professional soldiers. The state shifted the burden of supporting the armies onto local populations and during Leo VI (r. 886–912) wove them into the tax system, with provinces evolving into military regions known as themata. Despite many challenges, Warren Treadgold states the field forces between 284 and 602 of the Eastern Empire were the best in the western world while Anthony Kaldellis believes during the conquest period of the Macedonian dynasty (r. 867–1056), they were the best in its history.

The military structure would diversify to include militia-like soldiers tied to regions, professional thematic forces (tourmai), and imperial units mostly based in Constantinople (tagmata). Foreign mercenaries also increasingly became employed, including the better-known tagma regiment, the Varangians, that guarded the emperor. The defence-orientated thematic militias were gradually replaced with more specialised offensive field armies but also to counter the generals who would rebel against the emperor. When the Empire was expanding, the state began to commute thematic military service for cash payments: in the 10th century, there were 6,000 Varangians, another 3,000 foreign mercenaries and when including paid and unpaid citizen soldiers, the army on paper was 140,000 (an expeditionary force was 15,000 soldiers and field armies seldom were more than 40,000).

The thematic forces faded into insignificance—the government relying on the tagmata, mercenaries and allies instead—and which led to a neglect in defensive capability. Mercenary armies would further fuel political divisions and civil wars that led to a collapse in the Empire's defence and resulting in significant losses such as Italy and the Anatolian heartland in the 11th century. Major military and fiscal reforms under the emperors of the Komnenian dynasty after 1081 re-established a modest-sized, adequately compensated and competent army. However, the costs were not sustainable and the structural weaknesses of the Komnenian approach—namely, the reliance on fiscal exemptions called pronoia —unraveled after the end of the reign of Manuel I (r. 1143–1180).