The Long March
To understand how the Komnenoi united the steppe tribes they passed on their Long March, it is first necessary to understand the nature of what they built, and in particular, what it was not. Specifically, it was not a territorial state as understood in the rest of Eurasia. Mapmakers in China and India might bring our their biggest brushes, and paint a vast swathe of Asia - up to a twelfth of the land surface of the Earth - in Roman purple. But this did not indicate, as to-all-appearances-similar expanses of dye did in other places, the existence of the heavy infrastructure of civilisation: Roads, canals, fortresses, walled cities. Nor yet did it indicate the less visible parts of an empire: There were no tax collectors, garrisons of regular soldiers, official records of ownership or marriage or inheritance, or state church. Of the four armies that other nations considered essential to stability - a standing army of soldiers, a sitting army of bureaucrats, a kneeling army of priests, and a crawling army of informers - the Komnenoi kept only the first; and even then, since that army in effect consisted of every Roman male of fighting age, it was not a regular army as that term is understood in countries based on the ownership of land.
The new Roman state, then, was in one sense the most fragile of constructs: It existed only in the minds of men. In particular, the state consisted of the continuing choice of a thousand tribal chieftains, in consultation with their foremost warriors, to remain loyal. Now to an extent this is true of all states; even the worst tyrant must, at a minimum, keep the loyalty of his inmost circle, at least to the extent that they do not draw their weapons and kill him while discussing the fate of a million peasants. But in the Komnenos Khanate this fact was exaggerated to absurd lengths. Any chieftain, if he found a yearly tribute onerous, or a levy of horses or fighting men inconvenient, or even if he merely disliked the manners of the envoy sent to treat with him, might simply remove himself a thousand miles, or ten thousand, into the trackless steppe, bringing his herds and fighting men with him; and what then was a central authority to do? True, good grazing and fertile oases are not found everywhere on the steppe, and most tribes recognise at least a rough division into traditional ranges; so the power of the chiefs to resist unwelcome demands of the state was not completely without limit. But it was far greater than that possessed by anyone whose wealth and power depended on farmland. Even a great feudal magnate, who behind strong castle walls could defy - for years on end - the armies of his king, could still have his fields burned or confiscated, and find himself impoverished even though plague or invasion might lift the siege and force a reconciliation.
How, then, did the Komnenoi build a state from the disparate and fractious steppe nomads, and a state, at that, which militarily was a match for anciently civilised and densely settled powers such as China? First, we must note that humans habitually form hierarchies, and the nomad tribes were no exception; by long-standing custom the chief of one tribe or another was recognised as Khagan, and thus exercised at least nominal hegemony. The office was rarely hereditary, shifting with fluctuations in the herds and the fighting tails of each tribe, and there was not always complete agreement on just which leader was Khagan; but there existed, at any rate, the rudiments of a supertribal framework; it was not necessary to introduce the very concept of hegemony - Genghis and Tamerlane, although defeated in their European campaigns, had managed at least that much.
Second, although they had fled Anatolia with only what they could carry, in nomad terms the Romans were actually quite wealthy. Specifically, they owned several thousand excellent horses, in addition to a great quantity of iron weaponry, armour - and cookpots! True, these goods were effectively irreplaceable, and in using them the Romans were expending capital, not income - but the point remains that they had a great deal of capital, by the impoverished standards of the steppes. The five thousand trained kataphrakts that were all that remained of the legions were an immense advance in striking power and even tactical mobility over what an average nomad tribe, or even a dozen tribes together, could muster. Iron weaponry and armour against bone and leather; immense grain-fed cavalry horses [*] against scrubby ponies; and a system of discipline that did not rely on the bonds of family and clan - these all added up to make the Komnenoi a formidable force, easily capable of overcoming any tribe or any coalition of tribes that could be mustered quickly. Of course it is ridiculous to suppose, as some writers have done, that five thousand cavalrymen might have overcome the combined forces of the steppe tribes; but now the lack of state infrastructure cut the other way, for who was going to raise such a coalition? There was always some line of fracture, some blood feud or grudge or vendetta, for the single most powerful military force to take advantage of.
Third, and perhaps most powerful (for this war of unification, ultimately, was waged in the hearts of men more than on the field of battle) the Romans offered a unifying ideology. The nomad tribes had always practiced a rough egalitarianism as far as adult males were concerned; but Roman propaganda - and it must be remembered that the practice of organised propaganda for state purposes was also new on the steppes, and the tribes had no acquired immunity to it - raised the equality of fighting men to the level of an overarching claim to superiority over the settled peoples. The Komnenoi claimed that Rome had been destroyed because it espoused the equal freedom of all citizens, which was to say all men able to bear arms; that the nomads were now the last bastion of this ancient principle; and that it was the duty of all to keep alive the fragile flame of liberty against the slave empires that would impose feudalism or, worse, bureaucracy on everyone. This brilliant piece of manipulation was carefully calculated to appeal to every faction on the steppes, from the Cossacks of the Russian border, whose founding myth involved fleeing from serfdom, to the half-sinicized Mongols of the far east, who had a long and resentful history of Chinese scholar-poet-officials attempting to impose Confucian ideals upon them. To assert the brotherhood of Rome - the epitome of agricultural empires, and the home of mass chattel slavery! - with the nomads who had sacked so many of its cities was, in the words of one historian, "too awesomely mendacious to fail"; however that may be, the nomads united almost as fast as Alexandros could march his refugees past their territories.
This is not to say that the feat was accomplished entirely without fighting; some tribal federations did resist, and some indeed maintained their resistance for decades and centuries, on the marginal subarctic edges of the steppe - tolerated by the Komnenoi partly as a demonstration that fighting men really did have freedom, not merely the freedom to submit to Rome; and partly because subduing them would be more trouble than it was worth. After all there is steppe and steppe; the Komnenoi and their network of tributary allies commanded the best grazing, the fertile oases, the convenient caravan routes - in short, such sedentary wealth as the steppe offers. Reducing every last marginal borderland to formal obedience was quite unnecessary.
The travails of the Long March rapidly became a founding legend of the new Rome, told - even while the marchers were alive, in some cases - in the same tones used for recounting the story of Romulus and Remus. But it is worth noting that the legend does not, with a few exceptions, focus on battles; the Romans overcome hostile weather, mountain ranges, endless distances, and internal dissent, but after Jvris Ugheltekhili, few external enemies.
So much for the unification itself; we may also ask, how was the fragile structure of loyalty maintained? As noted, the heavy cavalry horses and iron weaponry of the Komnenoi were capital, to be husbanded carefully and expended reluctantly. Like any successful and wealthy warband, they attracted recruits from other tribes, mounted and armed more traditionally, and thus rapidly constructed their own swarm of light cavalry to supplement the kataphrakts and infantry. But while this was a welcome addition to their strength, it was, ultimately, the same means that previous Khagans had used to maintain their hegemony - in other words, if the heavy cavalry were allowed to attrite away, the Romans would become just another pony-mounted tribe, susceptible to the usual generational shifts in the Khaganate. To introduce a unifying ideology was one thing, but to make unification under the Komnenoi stick, it was necessary to maintain a strong military advantage against any likely combination of tribes.
To do so, the Romans took for their territory the doubtful lands surrounding the Great Wall of China, where nomad and farmer had pushed the frontier of settlement back and forth for millennia. This area does not offer the best sweet grazing, and the climate is atrocious even by the hard standards of the steppe, and thus no tribe was mortally offended by its seizure. However, the main concern was access to the two crucial goods the Romans needed to maintain their military edge: Iron and grain. With China split by dynastic revolt, the Romans could offer a bargain to the northmost tier of cities: Protection from nomad raids, in exchange for a tribute of the products of civilisation - in particular, the iron weapons and the feed required to keep the kataphrakt tradition alive.
The Komnenoi were, for a short period, something altogether new in the world: A nomad tribe with some of the trappings of civilisation, in particular a splendid heavy cavalry. Under the leadership of Alexandros, they exploited this uniqueness ruthlessly: They positioned themselves perfectly between nomad and settler, gaining strength from both sides - nomad light cavalry for scouting and skirmishing, and settler grain and iron to maintain a core of heavy cavalry that allowed them to overawe the nomads. Thus each part of their strength supported the other, a closed virtuous cycle. No other tribe could have done it, because no other tribe had the initial capital of horses and heavy-cavalry training (a very different set of skills from light-cavalry skirmishing) required; and no other band of settlers would have done it, because it required abandoning homes and cities for the harsh life of a nomadic tribe. When Alexandros announced that his winter camp would lie in the shadow of the Great Wall that separates and unites nomad and farmer, the symbolism was clear to all.
[*] These were something of a double-edged sword, for unlike nomad ponies they could not survive by grazing alone; the Romans dealt with this by marching along the southern border of the steppes, where they could buy grain from the edge of cultivation.