pothos.org

Here is a question for the tactics geeks out there: do we have any source for cavalry using rhombus formations outside of the text of Asclepiodotus, Aelian, and Arrian's tactica? Polybius does not seem to remember that formation when he criticized Callisthenes' description of the battle lines at Issos although he was definitely suffering from selective memory when he wrote that passage ...

Technical changes were a big part of why Alexander and his generals were so succesful, but the histories are not very interested in the details Maybe if we had the first two books of Curtius Rufus, or a full-scale history of Philip rather than just part of a book of Diodorus ...

I don't really have time to research and discuss this subject, but being one of the "Tactics geeks" Sean refers to, I'll make a couple of brief comments. The Hellenistic manuals describe cavalry squadrons forming up in rhomboid formations as a 'Thessalian' tactic ( with an alleged attribution to Jason of Pherae, though it seems probable the tactical formation went back further according to Arrian 'Ars Tactica' 16.....). Given the details we are given, such as exact order of the formation, titles of the officers and where they were positioned, advantages of this formation etc, there can be little doubt that it was a real formation used in battle, not just a hypothetical/theoretical one invented by the manual's authors.

The Macedonians are not described as using 'rhomboid' formations, but rather 'wedge' formations for each squadron. This tactic was adopted by Philip II from the Thracians, who in turn borrowed it from the Scythians. Since our sources are concerned with Alexander and his doings, we are never told, save in the most general terms, what Parmenion and the Thessalians and others of the left flank actually did in Alexander's major battles, let alone what formations they adopted - though the manuals specifically say that the Thessalians 'usually' used the Rhombus..

As well as individual cavalry unit/squadron 'wedges', on occasion a mass 'wedge' is described, such as that of Alexander at Gaugamela.
" he himself for a short time led on his army in column; but when the cavalry had been sent off [under Aretes] to help those who were trying to turn the Greek right and had left a gap in the Persian front line, he wheeled towards the gap, and making a wedge of the Companion cavalry and the part of the phalanx which was at this point, he led them on at the double....." [Arrian III.14.1-2].

Alas, at Issus, we are not even given this level of detail. Arrian [II.10.6], for example, simply says that Alexander's right wing :
" ..... once within missile range, first Alexander's suite and himself too, in his post on the right wing, took to the river at the double, in order to confound the Persians by the speed of their attack, and by joining in the melee the sooner to receive the less harm from the Persian archers. All fell out as Alexander had guessed. For the moment the battle was joined, the Persian left gave way; and here Alexander and his immediate following scored a triumphant success....."

There is a possible inference ( Alexander and his suite/escort led the charge) that this too was in 'wedge' formation.

But if we are given little information about Alexander's own formations, it is hardly surprising we are told nothing of the Thessalians on the left: "..However, the Persian cavalry posted opposite to the Thessalians did not keep their ground on the river bed in the action, but crossed manfully and charged the Thessalian squadrons, and here there was a desperate cavalry fight; the Persians did not give way till they saw Dareius in flight ...."

Other battle descriptions in our sources are in similar vein, with a broad brush approach, and lacking in detail, so I doubt if having more of Curtius or Theopompus would help, since the relevant sections of Curtius and Diodorus for Issus and Gaugemala are even vaguer than Arrian........

We should not forget that our sources are general histories, not military histories or technical manuals.

The details that Sean and most modern historians crave are just not there, other than the tactical details we can glean from the technical Hellenistic manual [in all its three versions].