Heraclius created a new administrative system to try to defend the remaining Byzantine territory: themes. He began by seizing lands from wealthy landowners and monasteries in Asia Minor, then using the seized land as the basis for new territories from which to recruit soldiers. A theme was a territory, originally about a quarter of the empire in size, organized around military recruitment. A single general appointed directly by the emperor controlled each theme. In turn, only soldiers from that theme would serve in it; this led to local pride in the military prowess of the theme, which helped morale. It was only because of the success of the themes that Byzantine losses were not much worse, considering the strength of their foreign enemies. Eventually, the themes changed further into self-sustaining military systems. Soldiers were granted land to become farmers. From there, they were to fund the purchase of weapons for themselves and their sons. Young men still joined the army, but the system could operate without significant cash-flow from the imperial treasury back in Constantinople.
In essence, the theme system was a return to the ancient manner of military recruitment that had been so successful during the days of the Roman Republic: free citizens who provided their own arms, thereby relieving some of the financial burden on the state. At their height, the themes supported an army of 300,000 men (comparable to the Roman army under Augustus), with the financial burden evenly distributed across the empire. The four themes were divided over the centuries, with villages being watched by commander and people fighting directly alongside their neighbors and families. Ultimately, it was this system, one that encouraged morale and loyalty, that preserved the empire for many centuries. One straightforward demonstration of the strength of the system was that the perennial enemy of Rome, the Persians, fell against the Arab invasion of the seventh century while the Byzantines did not.
There is an important caveat regarding the consideration of the themes, however. While Byzantium did indeed survive as a state for many centuries while neighboring empires like Persia fell, Byzantium itself arguably ceased to be an “empire” by the middle of the seventh century CE. The Arab invasions swiftly destroyed Byzantine power in the Near East and North Africa, and while fragments of Justinian’s reconquest remained in Byzantine hands until the eighth century, “Byzantium” was basically synonymous with the contiguous territory of the Balkans, Greece, and most of Anatolia by then. It was, despite its continued pretensions to empire, really a kingdom after the territorial losses, peopled almost entirely by Greek-speaking “Romans” rather than by those Romans as well as its former Syrian, Jewish, African, Italian, and Spanish subjects.