How did Alexander found so many cities? Where did all the settlers come from?

While we call them “cities”, Alexander’s cities were mainly frontier outposts with small garrisons to hold the territory, usually along preexisting trade routes or strategic locations. You have to remember that Alexander spent almost no time actually managing his empire, most of his energy was directed at further conquest. To enforce his authority, protect his supply lines across essentially unfriendly territory, and control the tax base he needed many small to medium garrisons. These many small garrisons (“cities”) actually rebelled against royal authority when word reached them that Alexander was dead, in total gathering a force of only 10,000, despite controlling vast areas of Asia. The soldiers of these garrisons had Asian wives and children, as well as merchants and businessmen who set up shop to service the new outpost. Eventually trade routes would connect these cities to each other and eventually back to Greece, creating lucrative trading opportunities and creating cultural and scientific exchanges that attracted settlers. There’s an interesting anecdote from Dividing the Spoils by Robin Waterfield that the years of Alexander’s empire and the early decades of the Successor kingdoms were a renaissance in Greek theater, the art form being so popular that acclaimed Greek actors were found as far away as Samarkand in Bactria.

Greek colonization was also not a new phenomenon, for hundreds of years Greeks had gained experience founding hundreds of colony cities across the Mediterranean and Black Seas. This wave of colonization provided a ready pool of experience vis a vis founding cities. Colonies were started for business opportunities or political freedom/exile, and combined with overcrowding on the rocky Greek mainland, fostered an attitude toward settlement abroad as a positive social/political expedient. Alexander’s empire opened up huge new areas of the world to any Greek brave enough to try their luck. Not to mention the automatic social status you would get as a member of the new ruling class (because no conquerer lets the natives be their equals). By the end of their long campaign even the poorest Macedonian soldier was still incredibly wealthy by the standards of the time, and the lure of easy riches is a tale as old as time. Get it wrong and you’re still probably better off than in Greece proper, get it right and you’ll live a life you could only dream of in Greece.

This colonizing attitude was reflected in Alexander’s own army. At least 10,000 of his men were Anatolian Greeks settled in Ionia who were placed there by his father Philip years earlier in preparation for his own invasion of Persia. Almost a quarter of his army were colonists themselves or products of the colonial experience.

Another incentive to settlement was the kleruchoi system. This was a land grant system that gave Greek settlers land in exchange for military service and tax enforcement. It was originally offered to mainland Greeks by earlier colonies as a way to ensure Greek superiority over locals, and to entice further military settlement in trouble spots. The Macedonian empires easily adapted this system to their own needs. Alexander and the Successor kingdoms had land to spare, restless natives, and manpower issues (because they couldn’t trust the native population, Successor kingdoms relied on Greek soldiers). So they offered land, training, weapons, and a new social status to any Greek who would accept. This system allowed the Seleucid and Ptolemaic armies to consistently field large armies of mainly Greeks, with Asian/Egyptian auxiliaries, despite losses from 50 years of constant war.

As far as native resentment, it depends. Alexander rarely removed Persian satraps after assuming control of their territory, so for many former Persian subjects, not much changed. For many the Greek settlement was profitable (think trade again), and if (when) Greeks eventually mistreated their native populations, by the time the locals realized it, the cities had already grown and been reinforced, fulfilling their original purpose. Over time however native resentment absolutely grew, and each Successor state took a different approach toward the locals. The Seleucids adopted local customs and became more Persianized, therefore largely avoiding native rebellions. The Ptolemies segregated their populations and forbid native Egyptians from govt and the military, leading to both Greek manpower shortages in later years and several very serious native rebellions, including one that seized control of half of Egypt for almost twenty years.

All in all there’s no telling how many Greeks left Greece for these colonies (Is be very interested myself to see any numbers estimate, no matter how rough) but the consistent size of almost purely Greek armies used by the Successors indicates it was not an insignificant number. Add in the significant Hellenizing influence Greeks had on the cultures of the Successor states and it’s clear that while always a minority, Greeks were by no means rare in Alexander’s empire. The incentives were plentiful, the culture was accepting of settlement and had the experience. Native resentment wasn’t the serious issue it would later become, and there was already a system in place for military settlement to ensure local compliance.