I've had a strange day today that's led me to drinking scotch and reading about WW2 era tanks which turned into me falling down the Wikipedia rabbit hole of related topics. My journey brought me to the page on the Law of War which contains the following paragraphs about historical laws of war:
In the Indian subcontinent, the Mahabharata describes a discussion between ruling brothers concerning what constitutes acceptable behavior on a battlefield, an early example of the rule of proportionality:
One should not attack chariots with cavalry; chariot warriors should attack chariots. One should not assail someone in distress, neither to scare him nor to defeat him ... War should be waged for the sake of conquest; one should not be enraged toward an enemy who is not trying to kill him.
Now most of that makes sense to me, there are some basic principles about who and why you should and should not attack, and a quick google search tells me that the basic principle behind the Rule of Proportionality is that you should not attack a military objective when the attack would risk excessive civilian casualties.
When I read the page on the The Mahabharata it switches the statement around instead saying that chariots shouldn't attack cavalry:
The Mahābhārata offers one of the first instances of theorizing about dharmayuddha, "just war", illustrating many of the standards that would be debated later across the world. In the story, one of five brothers asks if the suffering caused by war can ever be justified. A long discussion ensues between the siblings, establishing criteria like proportionality (chariots cannot attack cavalry, only other chariots; no attacking people in distress), just means (no poisoned or barbed arrows), just cause (no attacking out of rage), and fair treatment of captives and the wounded.
Like the previous quote a lot of this makes sense to me, they are discussing some basic rules of conflict to try to create a "just war" but again the idea that chariots and cavalry should not fight each other comes up.
This brings us to my question, why is there an apparent stated prohibition on chariots and cavalry fighting each other?
First of all, Mahabharata is an epic which has evolved through several millennia to its current state of a semi-religious/historical scripture. So, there are underlying religious assumptions that are required to answer this question; in that sense, the answer isn't going to be strictly historical, because Mahabharata, and the question, are not.
It is important to understand some religious undertones and some Hinduism terminology first. The war of Kurukshetra, which is the main war Mahabharata describes, has been said to take place at any time between 1000-4000 BC, date varying by several historians. More importantly, speaking from a religious perspective, it takes place at the end of the Dwaper Yug (Yug = Eon/age; there are 4 Yugs in Hinduism, Satyug, Tretayug, Dwaperyug and Kaliyug). Over the course of the 4 yugs, humanity is said to undergo a slow moral decline, with Kaliyug being the most corrupt age of all. Here is a paragraph describing the state of society during Kaliyug:
People become sinners and lack virtue. They become slaves to their passions and are barely as powerful as their earliest ancestors in the SatYug. Society falls into disuse and people become liars and hypocrites. Knowledge is lost and scriptures are diminished. Humans eat forbidden and dirty food. The environment is polluted, water and food become scarce. Wealth is heavily diminished. Families become non-existent. By the end of KaliYug the average lifespan of humans will be as low as 70 years.
The scriptures mention that at the end of Kaliyug, the tenth avatar of Vishnu, Kalki (hence the name Kaliyug) will appear to cleanse the world of evil and usher it into a new Satyug. (As a side note, Hinduism considers time to be infinite and universe to be cyclical; the current universe is just the start of a present cycle preceded by an infinite number of universes and to be followed by another infinite number of universes. The scriptures say that after a thousand of such 4 yugs, the Harihara (creation/ driving force of universe) will cease to exist, and Shiva (the destroying force) will preside, reducing the universe back to a singularity, from which another universe shall emerge to continue the cycle).
Back to the question; the war of Mahabharata marks the end of Dwaperyug and beginning of Kaliyug. These rules that are set in place to carry out a 'just war' (the word 'just' being applied as a whole to the entire war, as well as each individual combatant and each individual combat). It is assumed that the fighting prowess of a chariot and the cavalry are different (whether or not the difference is significant is irrelevant), and thus, it is not appropriate for them to directly face off against each other, because one of the combatants will have an unfair edge- however small it might be. Mahabharata goes into extreme details of the war, often describing each individual combat of all the main and side characters, and for the most part, they are all fighting each-other on chariots/ cavalry/ with the same weapon.
It's important to note that because this war marks the end of a more pure and honorable age, near the end of the war, several rule violations were observed, even by some of the most righteous characters, harbingering the upcoming age of corruption. The end of the war, and the beginning of the Kaliyug, was followed by an assassination on the winning side which left them with only 6 survivors.
So to summarize, the point of the rules, more or less, is to highlight the increasing corruptness of humanity (to compare, another epic at cornerstone of Hinduism, the Ramayana, takes place during the Satyug, and no 'just war' rule violations are seen; in fact, they are not even stated, just implicitly understood by every combatants).
Of course, this is all based on the epic. The real war of Kurukshetra (and ancient South Asian wars), whenever it took place between 1000 and 4000 BC, most likely, did not observe such rules.