From the founding in AD 330 by Constantine I the Great up to the death in battle of Constantine XI Palaiologos in 1453 on the day Constantinople fell to the Turk. Well over 12 centuries!
It wasn't a stable millennium. Their borders swelled and ebbed. There were sixteen different dynasties and several periods of internal instability. Their capital city was sacked and occupied for 50 years by Western crusaders supported by and urged on by Venice during the Fourth Crusade. The invaders were aided by internal dissension. But even then Byzantines survived in three successor states east and west of Constantinople and eventually liberated it.
How did they survive so long?
Edward Luttwak’s book “The Grand Strategy of the Byzantine Empire” touches on some of the reasons why. Interesting read if you are fascinated by the duration of empires, dynasties, republics, institutions and such. Luttwak elaborates on seven major facets pf Byzantine strategy that may answer some of the reasons why they survived so long:
I. Avoid war by every possible means in all possible circumstances, but always act as if it might start at any time.
II. Gather intelligence on the enemy and his mentality, and monitor his movements continuously.
III. Campaign vigorously, both offensively and defensively, but attack mostly with small units; emphasize patrolling, raiding, and skirmishing rather than all-out attacks.
IV. Replace the battle of attrition with the “nonbattle” of maneuver.
V. Strive to end wars successfully by recruiting allies to change the overall balance of power.
VI. Subversion is the best path to victory.
VII. When diplomacy and subversion are not enough and there must be fighting, it should be done with “relational” operational methods and tactics that circumvent the most pronounced enemy strengths and exploit weaknesses.
Don't know much about Luttwak and have no idea if he knows what he is talking about. His companion book on the grand strategy of Rome was criticized by many historians. And he was seen by some as a neocon, although he was reportedly against the invasion of Iraq and against bombing Iran.
Several other reasons that the Byzantines lasted so long. Most included in Luttwak's book, some in detail, some briefly. Others are speculation on my part (or perhaps I remembered them vaguely from FDChief's excellent blogpost on the fall Constantinople two years ago?).
- geography - They sat astride the trade routes, both the East/West routes and the North/South routes. This made them a commercial powerhouse. A treasury full with gold buys a lot of friends and allies, buys off a lot of potential adversaries, and pays a lot of soldiers and sailors (and provides for their equipment).
- navy - They dominated the Black Sea and the Eastern Mediterranean for more than five hundred years, and even the Western Mediterranean early on. Their fleets managed to hold off the Arab fleets in the Seventh and Eighth centuries. Eventually they lost naval dominance to Venice and Genoa and later to the Turks resulting in disaster.
- legacy - They had a military legacy from ancient Greece and the earlier western roman empire. Not strategy, but they took a lot from their forebears on military organization, training, tactics, operational methods, and in the means of evaluation of different strategies
- engineering - This was another inheritance passed down from Rome. They carried on with the advice of Domitius Corbulo: that the dolabra (a combination pickaxe tool) “was the weapon with which to beat the enemy". The walls of Constantine and of Theodosius are testimony to that, and the hundreds of cisterns they built for when the aqueducts failed during a siege.
- tax revenues – Tax collection was rigidly organized and sophisticated. It was a very effective system. No other contemporary powers could compete. It filled their treasuries and gave them a huge advantage.
- bureaucracy - They had a capable and enduring bureaucratic class. It was they who administered the empire, guided diplomacy, counted beans in the treasury, organized and oversaw military logistics and training. They provided the continuity and institutional memory needed through those sixteen different dynasties and 96 emperors/empresses. Without them - chaos with each change of crown.
- land for service in the army - This put tens of thousands of veterans on the frontiers of the empire. Their family's safety gave them incentive to band together into ad hoc militia units. They retained their weapons.