Byzantine Faction History Overview: A Short History of Late Medieval Roman History
The medieval Roman Empire, commonly known as the Byzantine Empire, was a clear continuation of its classical Roman past. It is possible to trace a continuous line from the legions of the classical empire to certain of the military divisions of the medieval empire, the Senate of New Rome (Constantinople) was at one and the same time the equal partner and the replacement of the Senate at Old Rome, and the imperial bureaucracy operated without interruption, even following the temporary loss of Constantinople to the 4th Crusade.
Understanding the empire requires an accurate understanding of its past and of its continuity with the classical empire. Its age lent it unsurpassed prestige and an aura of power and influence that outlasted its actual practical ability to exert force, while its military traditions ensured that so long as it retained the ability to pay its troops its armies would have the potential to be the best in the world. Its rich tradition of military evolution and adaptation only halted when its money ran out; innovations like the adoption of the stirrup (c the late sixth century), the invention of Greek Fire (roughly equivalent to modern napalm, though liquid rather than gelatinous in consistency), and the willingness to employ a mixed force of disciplined cavalry and infantry, including both heavy lancers and armored horse archers, all illustrated the empire's capacity for military development. Even when gunpowder was first introduced the empire attempted to keep pace, with a simple lack of funds eventually dooming it to fall irrevocably behind.
The most immediate historical cause for the situation that the late medieval empire found itself in would be the events surrounding the infamous battle of Manzikert. For over a century prior to that battle, the empire and its armies were essentially unstoppable. With emperor-generals such as Nikephoros II Phokas and Basil II Bulgaroctonus, the imperial armies were unmatched in skill, discipline, and morale. Unfortunately, Basil II failed to make provision for the succession, and so a series of lackluster rulers followed. From that point on the army was neglected, either falling into disuse or actively being decommissioned (particularly the veteran Armenian troops, whose military obligations were commuted to cash payments). This brought to empire to the brink of disaster in a matter of only a few decades, with the only reason for its survival being the reputation that the great emperors of the proceeding years had earned for it. Emperor Romanus IV Diogenes c1068 decided to fix the problem. He began a series of sweeping military and administrative reforms intended to rebuild the failing Theme system and revitalize the army. Unfortunately, he moved a little too fast and he faced dangerous opposition from the Dukas family. His rebuilt army met with continuous success, but was still fragile. He moved to engage the Turkish army in western Asia Minor c1071, and disaster struck. At the battle of Manzikert his Norman mercenaries abandoned him and Andronicus Dukas, the aristocrat in command of his second line (standard imperial deployment was in at least two lines, separated by anything from a bowshot to a mile of distance, depending on terrain), began to spread the false news that the emperor had been killed in battle. This resulted in the emperor, along with the entire first line of his army, being surrounded by the Turks while the second line fled headlong. Romanus IV made a generous peace treaty with the Turks (likely because the Turks were more focused on Egypt than on Asia Minor), and would have been able to resume his reforms had Andronicus Dukas' faction betrayed and murdered him upon his return to Constantinople. This resulted in a civil war that allowed Turkish adventurers and raiders to occupy most of the central Anatolian plateau, which was prime horse country and important to the empire's economic and military might.
The civil unrest continued until Alexios I Komnenos seized power and took firm control of the state (c 1081). He managed to rebuild the army (several times, due to invasions by Normans from Sicily and Pechenegs from north of the Danube) into an effective, even feared, fighting force, though this time it was founded on grants of pronoiar rather than on the former thematic land grants. The crucial difference between the two was a matter of taxes. Thematic land grants were grants of income, but the income was still taxable. Pronoiar grants were grants of tax revenue, thus denying the state any and all income from the lands involved. This made the pronoiar system in the long run a very dangerous one, similar to a poor man selling too much of his blood to various blood banks; it made him some cash in the short run, but over time it bled him to death.
Even with the financial problems caused by the pronoiars, the empire was on the road to recovery until Manuel I Komenos walked into an ambush and got his army annihilated. This issued in a period of military decline that culminated in the rise of the Angeloi, a family more interested in civil war than anything else. Their scheming eventually brought about the disastrous 4th Crusade, the only time that the city of Constantinople fell to a non-Roman attacker since the day of its founding. The imperial administration remained operational from the capital in exile in Nicaea, until 1261 when Michael VIII Palaeologus retook the City. The revitalized empire was unable to last, however, because the Ottoman Turks had become too powerful. With the loss of the last of their territories in Asia Minor the empire was reduced to the status of client to the Turks. Frequent civil war also hampered the empire's attempts at recovery, wasting both men and money in pointless fighting. Several attempts by the Hungarians and other Balkan states to lead Crusades to rescue the ailing empire failed, and finally in 1453 the City fell to a Turkish assault, with the last emperor disappearing among his men, presumably dying in the defense of the breach (Constantine XI Palaeologus, who when he saw that the breach had been made removed his imperial insignia, drew his sword, and rushed with his bodyguards and retainers to defend the City to the last).
In the end, the main sources of decline for the empire were monetary. So long as it retained sufficient income or sufficient cash reserves to retake income-producing lands it was always able to weather incredible disasters. Once it lost its richest territories and once the cost of defending its lands outweighed the income from them it quickly fell into decline. Even so, its last centuries saw some of the greatest flowerings of cultural development in its entire history.
Suggested reading for those interested in more detailed information about the empire should look to the works of Warren Treadgold (Byzantium and Its Army, 284-1081; A History of Byzantine State and Society), John Haldon (Warfare, State, and Society in the Byzantine World, 565-1204), and various of the Byzantines themselves (from books on tactics and strategy to books on administration and ceremony, as well as a wide variety of histories, chronicles, and theological treatises). Especially look to Anna Komnena (Alexiad) and Emperor Constantine VII Porphyrogenitus (On Ceremonies; On the Themes; On the Administration of the Empire). For earlier periods look to Procopius (The Gothic/Vandal/Persian Wars; The Buildings; The Secret History [take this with a large grain of salt]), the Emperor Maurice Tiberius (The Strategikon), and one of many Georges, though I can't remember which exactly, of Pisidia perhaps, who wrote a chronicle of the campaigns of Heraclius I the Great.