Alexander the Great’s success depended on the fact that the Persian Empire was there for the taking:
The Empire was already an organized entity. Cyrus, Darius I, and their successors had created a unified administrative structure out of what previously had been several major kingdoms (Media, Babylon, Egypt), plus lesser kingdoms, plus a vast area that never before had been a state in the strong sense of the term. Back in the time of Cyrus in the 500s BC, Mesopotamia and Egypt, the two great fertile river valleys of the Middle-East, had already gone through their elimination contests and winnowed down to a few strong states based on big populations held together by water transport. But Iran, the uplands of Asia Minor and Armenia, and the adjacent plains of Central Asia, were still areas inhabited by sparse populations. Some were moving pastoralists, who formed at most shifting tribal coalitions. Others lived in pockets and valleys where agriculture could support a mid-size population and therefore petty kingdoms; but they lacked the logistics to supply an army big enough to conquer anybody, by carrying enough food and water to get across the infertile areas between them.
What Cyrus did was essentially what Alexander did later: starting from the major pockets of population and agriculture, he would win a few exemplary victories, then use his prestige to invite or overawe the outlying areas, with their lower level of production, to enlist as friends and allies. We could call this a system of tribute; the Great King, as Cyrus and his successors were known, was more than just an ordinary King, but overlord of lords. He did not change much locally; the same chiefs and petty kings remained in place, but they had to pay tribute. Above all, they had to provide goods in kind, especially the animals and foodstuff so that royal armies could pass that way.*
*In this respect, the expansive emperors, Darius and Xerxes, regarded the Greek city-states of Asia Minor and the other side of the Aegean sea as just so many more candidates for incorporation into the system of overlordship. Greek historians, and some contemporary politicians, saw this as a life-or-death struggle between democracy or despotism, but this was an exaggeration. From the Persian point of view, the Greek city-states were a version of small remote kingdoms, too much trouble to be directly controlled. The city-states of Ionia under Persian overlordship were left to run their own internal affairs; some continued to be democracies, others were oligarchies but this was the same spectrum as the Greek mainland. On the whole tribute was light, in fact generally less than what the Athenians demanded to maintain the anti-Persian fleet.
This was a thin administrative system. In some places, a tributary empire could be turned into a thicker, more intrusive system. Cyrus, Darius, and their stronger successors put their own administrators in place: high-level satraps, intermediate level governors, local garrisons. In richer places, older city-kingdoms like Babylon, taxes could be collected in money for the royal treasury. Paved roads were built, facilitating the faster movement of armies to keep things under control; messengers connected administrator and sent policy edicts throughout the Empire. With only moderate success, to be sure; satraps were often near-autonomous; and since they ruled over layers of locals most of whose traditional leaders were kept in place, they often had little effect except keeping the taxes or tribute coming in. Under the stronger Persian regimes, regional power was divided among a civilian head of government, counterbalanced by a chief treasury officer, and a military commander. There also was a service called “Eyes and Ears of the King,” roving inspectors with their own military escorts.
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Sheer military force cannot take over a territory before it has developed to an economic level at which the conquering forces can be sustained. At the cusp of civilization, large armies couldn’t even traverse such places if economic organization isn’t complex enough. Conversely, a state with a strong enough infrastructure to support its military rulers also can support a conquering army. No Greek general, like Alexander or anyone else, could have conquered an empire spreading into the Iranian plateau and beyond into Central Asia, in the 500s BC when those places were still isolated agricultural oases amidst tribes and pastoralists. It required the intermediate step such as Cyrus took, to build the logistics networks.