Why didn't the Persians create good infantry units?

One problem of Persia was that it was not one ethnicity, but a conglomerate of many different small countries. Each of these countries produced their own armies and formations, led by their own officers; there was really no central "Persian" army. So Persian military strength was on paper only; hundreds of thousands of men in "national" militias but with no central command or unified tactical doctrine. The (biblical) Book of Esther refers to "an hundred twenty and seven provinces, unto every province according to the writing thereof, and unto every people after their language," in which military commands had to be issued to the troops.

There were a "few" countries that produced better infantry formations than others. But the Persians did not identify them and insist that other countries' units learn from them. The Greek city states spoke a common language, and were geographically close, so it was easy for them to "compare notes" and develop similar infantries.

The Persians did better with cavalry troops; there were fewer of them, and they had better "natural" transportation, and hence communication, so Persian cavalry developed some cohesion.

Another question is why did the Greeks develop good heavy infantry. Heavy infantry required a lot of discipline, mutual trust, and nerve--if part of the line broke, everyone's life was at risk (whereas in a cavalry rout individuals could still get away).

If you look at other good heavy infantry units in history, such as Flemish or Swiss pikemen, they all came from the same town/valley/canton. They knew and implicitly trusted one another, which made it easier to train them to act together. Likewise with the Greek city-state.

The Romans were the only pre-modern army, as far as I know, that developed a military machine capable of instilling sufficient esprit-de-corps in men from disparate regions. But they only did this because of their exceptional organizational abilities, which they used to scale up the infantry units of the earlier Italian city-states.

Now back to the Persians: much of their infantry was formed from auxiliaries from tributary regions. The antecedents of the core of their army were nomadic cavalry, which they scaled up into the most effective military force for thousands of miles. With such an outstanding specialty, and without any of the foundational skills to build a solid heavy infantry corps, it would be surprising if they ever did develop good indigenous heavy infantry.

The Persian army at Marathon was (as at Thermopylae) at a serious disadvantage because of the terrain, which favoured the much smaller defending force tremendously.

Their failure to win in both cases is more to blame on the leadership insisting on frontal assaults on a narrow front against a force trained and equipped specifically to deter such an assault rather than on the forces sent to fight that assault. A better commander would have recognised that and kept a blocking force in place himself while sending the bulk of his forces in an encirclement in order to outflank the Greeks.

Why the Persians elected not to do this is a matter of debate among historians for a long time. Maybe they'd grown overconfident after decades or more of military successes, maybe they were c**ky thinking those puny numbers of Greeks could never stand up to them, maybe they were under pressure to provide a quick victory.

Individually, I seriously doubt a Greek soldier was that much better than his equivalent in the Persian army. But the Greeks were employed much better, using the terrain to their utmost advantage where the Persians did not.

Remember also that both Greek victories came after heavy Greek losses in earlier engagements where similarly equipped and trained Greek units were wiped out by the Persians in pitched battles.

Question: Why didn't the Persians create good infantry units?
The defeat at Marathon, Platea, march of the 10,000, and the hold up at Thermopylae would really suggest the need for some heavy infantry that can fight on a par with the Greek Phalanx. How much of your infantry really needs to be very mobile? The battle of Gaugamela also displayed the ability of the Greek heavy infantry to deal with flanking maneuvers

Why didn't the Mongols or Huns develop better heavy infantry? It was the style with which they were successful, and that model (horse based archers) remained effective long after the phalanx had disappeared from use.

I would argue that the Persians were a very successful and very respected military empire. They had battled and defeated both the Assyrians and the Babylonians. They had held off the Egyptians and they even gave Rome all they could handle in latter years. You don't conquer a large empire like Persia without having a very good army.

Yes, their heavy infantry was inferior to the Spartans at Thermopylae and Marathon but that's hardly a knock on the Persians. The Spartans went centuries without one of their armies losing. The Spartans were basically a military cult. Their entire culture and way of life was based around training for war. They even defeated the Athenians.

The battle of Gaugamela doesn't support your thesis. At Gaugamela, the Persian infantry was engaged with Alexander's phalanx. Alexander was commanding the companion cavalry and it was the use of Alexander's cavalry wedge which defeated the Persian infantry rather than the phalanx. The Persians were engaged with Alexander's phalanx and he flanked them with heavy cavalry.

In fact Alexander's phalanx with its longer spears were used to hold enemy units in front of them in order for his cavalry or archers to attrite them. Again this is not a knock on the Persians. I don't think it's a sign of incompetence to lose to Alexander the Great.

Alexander's two big advantages were:

1. He commanded a full time professional army where other armies at the time were made up of farmers, or only part time soldiers. Alexander's father had built that Army and had trained Alexander how to use them. Their great innovation was not the Phalanx either.

2. The Other Macedonians innovation was they had mixed units. Where other armies consisted of predominantly one kind of unit, the spartans basically used the phalanx and that's it. The Macedonians (Alexander) had heavy infantry, light infantry, heavy cavalry and light cavalry. They were flexible. Alexander was the first to use field artillery, siege weapons modified to be used against infantry. The Macedonians could put units against their enemies which maximized their enemies weaknesses. Heavy infantry was just one of the kinds of unites in Alexander's army. Alexander himself fought commanding cavalry; his companion cavalry. (heavy cavalry)