I heard the Numidians had legionaries, or at least legionary style troops, despite being in the desert. Would heavy infantry actually serve any purpose in a desert climate, considering cavalry dominated the scene?
Yes, for a couple reasons.
First off, if you’re making a military, you want forces that you can take with you. Part of the problem for Numidia (and the Roman world) was you’d need to move units across water. It’s much easier to move humans, than horses. Water travel is very stressful for the animals, which can prove fatal.
Second, you can’t really make a standing military exclusively out of cavalry. Historically, armies that focused on cavalry faced serious threats from enemies which made extensive use of spear infantry. This is almost a hard counter. Polearms are the one thing a horseman does not want to face.
Infantry is still easier to move around strategically. I realize I’ve been throwing these terms around, and haven’t defined them recently, so let’s run over these:
In very general and reductive terms, there are three levels of warfare: Strategic, Operational, Tactical.
Strategic warfare is the entire campaign. It often includes things like large scale troop movements. The process where armies are moved around. If troops are being moved to take a city, that’s strategic.
Operational warfare is how you achieve your strategic goals. If you’re to take a city, you’ll need a way in. That could be a siege, it could be bashing down the doors, or it could be through subterfuge or other means. Those are Operations.
Logistics is a consideration at this level. Specifically, this is about keeping your troops supplied and ready to fight. (Also, why this might look like I just added a fourth tier, but, Logistics is a part of Operations.)
Tactical warfare is the ground level stuff. This is how you array your forces before the battle. Where your troops go. Who engages. Are you using your cavalry to protect your flanks, or are you using them to skirmish or flank your foes? Did you set an ambush?
Without sounding too much like a game, maintaining standing forces costs logistics (logistical resources, anyway.) You have to keep your soldiers fed, armed, and battle ready. Forces that are not fed and supplied are not under your control. Yes, they may fight for you when you ask them to, but you can’t stop them. You can’t prevent them from destroying the territory you just secured. You cannot reliably keep them from chasing after anything that’s dangled in front of them. They’re little more than bandits who are following your general suggestions at that point.
Cavalry is, logistically, significantly more expensive than infantry. You need to maintain the horses, and their equipment, in addition to the riders. Short version is: you can’t as many cavalry riders as you could infantry. There’s a real opportunity cost, where the resources you expend to field and maintain a rider could provide for multiple infantry. (I’m not sure what the exact trade-off numbers would be, because this depends heavy on many economic factors in the culture making the choice, and I’m hesitant to throw numbers around without any basis.)
We also just did the discussion about the tactical limitations of cavalry. They have significant advantage for dealing with enemy infantry, but as I said, they’re not an, “I win,” button. Even if you had a culture that made extensive use of cavalry, defensive positions, and assaulting fortified locations (like cities) would still require infantry support.
The Numdians made extensive use of light cavalry. I’m going to go out on a limb here, and say that fielding heavy and shock cavalry in the desert is probably extremely difficult. Simply as an environment and breeding issue. The same goes for heavy infantry.
Armor is exceptionally good at trapping heat. This can be a serious issue even in temperate environments. As the wearer fights and exerts, they’ll heat up, and the armor will, in most cases, prevent that heat from venting. Heat is exhausting, and in a prolonged battle overheated forces will start faltering much sooner.
I don’t know what the Numidian Legions looked like, but it would have been a factor for heavy infantry and cavalry. So, I’m not sure what they did, or how they dealt with those considerations.
The infantry is the backbone of an army. That’s as true in antiquity as today. For the time, the Roman Legions were the region’s gold standard of military forces, so it’s unsurprising that the Numidians (and other nations) emulated that structure, (with variations.)
So, yes, infantry is easier to field, easier to maintain, and useful in situations where your horses simply aren’t. Cavalry is a significant advantage, but it’s a specialized combat tool which excels at getting around and out maneuvering enemy forces. It’s not your only weapon, and unsupported it’s going to be forced into situations where it doesn’t excel. At that point, losses are significantly more expensive.