One point I must quibble with: The characterization of the Persian army. The Persian military machine was a conglomeration of various national contingents (for example, see Herodotus on the different contingents in Xerxes’ army, “The Persian Wars” VII.60-99). Many of these were excellent troops. Likewise, the Persians themselves were considered superb soldiers. Xenophon heaps praise upon the Persian military in his “Cyropaedia”, and Alexander himself hastens to add Persian troops to his army after the fall of the Achaemenids (the army that Alexander led into India was four times the size of the force that first crossed the Hellespont, whereas the flow of troops from Macedon was barely enough to replace losses in the Macedonian units). A close reading of Alexander’s battles reveals moments when Persian skill and grit nearly win the day (such as on the Macedonian left during the battle of Issus).

There is this a modern notion that the Persian army rarely rose above slapping the Greeks with their purses before dying. On the contrary, the Persians were tough hombres who built a mighty empire. The Greeks and Macedonians thrilled at their victories over Persia precisely because of this fact. There is no glory in defeating wimps.

Babylon’s massive metropolitan size worked against maintaining an effective defense – when Alexander finally entered Babylon it was not an ancient Stalingrad with spears but closer to Paris with the Wehrmacht marching by the Arc de Triumphe. I don’t think Alexander initially even bothered to wipe out all of the city’s former rulers ( some of that came later).

It is interesting that Alexander, based in part on the Anabasis, chose a different route.

Do we know what the scope was of Alexander’s other information gathering efforts? Did he send spies into the interior to map out a route? Did he question travelers? How formal was the process?

Another critical datum derived from the Anabasis is that a Greek army has much greater fighting power than anything the Persians could generate, man for man. Knowing that would give an invader a lot of confidence. Plus of course, Alexander not only had his Greek troops but his own Macedonians, who had conquered the Greeks. So if Greeks > Persians, and if Macedonians > Greeks — Conquering the Great King’s lands should be relatively easy!

The enormous army of Artaxerxes II would have quickly and dramatically depleted the surrounding land of all food, fodder, and perhaps even other supplies, such as firewood to cook with. Campaigning in Mesopotamia instead of Persia proper would have minimized the political fallout of this ancient scorched earth variant. It is easy to imagine that approaching an army of this size (a million men..) would be an advance through a desert stripped of all supply by this enormous force. Worse, the Persians might have elected to stand on the defensive, compelling Alexander to attack on their terms before his logistic problems became a logistic nightmare.

The Northern approach compels the Persians to shift from this strong defensive posture to an offensive one, which in turn requires them to divert a substantial portion of their forces to deal with their own logistics problem, and the force security issues that arise from the dispersion that this requires. This same dispersion of forces would allow the dissent alluded to above to further degrade the ability of the allied forces to act in concert. No doubt about it, Darius exerts much less affirmative control over his army in this scenario.

Advancing to the North of the Persians may have had benefits beyond the more favorable temperatures, the most important of which might have been the generally dry land. If the Anabasis has one glaring deficiency, it is that Xenophon reports few casualties to disease until the Greeks are based in Trapezus. The land between the Tigris and Euphrates at this time was likely a Malaria and Leishmaniasis infected swamp. The attrition from these diseases was likely horrific. I wonder if Alexander chose the more northerly route because he understood we would lose less of his relatively small army to disease if he did so.