Light (missile) infantry can successfully attack (hence the A) heavy (shock) infantry; they can each defend (hence the D) against light (missile) cavalry and heavy (shock) cavalry, respectively. Light cavalry can attack heavy cavalry and heavy infantry, and heavy cavalry is effective against light infantry. Light troops all wear little or no armour and usually fight with ranged weapons (bows, slings, javelins), whereas heavy troops fight wear (lots of) armour and fight using mêlée weapons, like swords, spears, and lances.
Naturally, this is an abstracted view of the relationships between different types of troops. Battlefield conditions obviously would have been incredibly important. Light cavalry would have a hard time against entrenched heavy infantry, or when they have to move and fight up-hill, or through marshy terrain. Similarly, heavy infantry would be at a disadvantage if they were scattered about too much. And so on.
The diagram is printed in the chapter on the “Medieval ways of war” (the starting date of which Jones places in AD 200, or shortly after the reign of the Roman emperor Marcus Aurelius). Indeed, for the ancient world, the distinction between “light” and “heavy” cavalry is indeed tricky, since a lot of the time the heavy cavalry wasn’t more heavily armoured than the light cavalry. The main difference lay in the weapon used (the lance versus the javelin or even the bow).
In fact, for much of the ancient world, a simpler diagram would suffice:
If you’ve ever played a strategy game inspired by ancient history, such as the classic Age of Empires (1997) or its later spin-offs, Age of Mythology (2002) and Age of Empires Online (2011), you’ll be familiar with this diagram or at least the relationship that it depicts.
It’s essentially a tactical form of rock, paper, scissors, where one type of warrior beats (counters) another. In this case, missile troops (archers, slingers, javelineers) beat shock troops (spearmen, swordsmen), who in turn beat mounted troops (lancers, horse archers, and so on).
While diagrams like the above obviously simplify historical realities, they do give an idea of what the relationships between different types of troops were on the battlefield. They also have the advantage of visualizing what seems intuitive: of course, armoured troops would be slower and more susceptible to missile fire. Of course, a unit of cavalry couldn’t charge directly into a line of men armed with spears or pikes.
As a study aid, such diagrams allow us to better gauge the tactics used in ancient battles, and the considerations that a general might have had about troop deployment. They also raise interesting questions. Where, for example, was the Persian cavalry at Marathon? Did they not deploy them for fear of the Athenian hoplites, or were other factors at play?
And by way of an update to this article, be sure to check out this piece by Roel Konijnendijk. In his PhD thesis, Roel emphasized the importance of cavalry and other kinds of fighters with respect to Classical Greek warfare, which is traditionally thought to have been dominated by the heavily-armed warrior known as the “hoplite”.