There is a tradeoff in fitness (a person can only train so much). If you want a stronger soldier, then they are gonna be worse in a different way, ie, lose some cardio. A person can only do, and maintain, so much.
True enough, but I wonder whether strength is more relevant now? I.e. they get helicoptered/driven to within 5-10 miles of their mission site with a ton of gear, rather than having to march 30 miles with their gear.
>rather than having to march 30 miles with their gear.
Not all of that is just physical conditioning, there's a sizable mental component to it that helps condition you to less than ideal situations so you don't go "welp, I'm tired, fuck it, I'm sitting down to die" when you're in a bad situation and instead you're like "I got this, I've done worse, this is nothing, just a little farther" as well as equipping you to know what your body is capable of and that the burning in your calves and quads isn't that bad yet and you know you've got to slow down for the next few minutes or that you've still got this much left in your tank and should be able to complete task without any rests.
There's a reason we readopted some of these conditioning practices from ancient militaries.
Off the top of my head the book Manthropology by Peter McAllister covers some of this, as far as what was expected of the ancient solider.
Also look into the history of Special Forces in the United States. WWI was in large part trench warfare, the Civil War was largely fought in open fields in lines not unlike trench warfare. Marching was something you did for parades and pageantry.
Basically though if you look at ancient armies like the Romans had you needed to be able to march 15-20 miles in full kit over most terrains and this is how you trained.
If you want to dive into the current Special Forces community you need to start with the First Special Service Force where you first start seeing long marches (on a 60 mile course) as well as learning things like hand to hand combat, skiing, mountain climbing, parachuting. Then go look at Darby's Rangers and Merrill's Marauders. From there jump to to Vietnam and the 5th SFG[A] and look at some of their feats.
Today you have stuff like Robin Sage and Operator Training Course which, special tactics aside, some ancient soldiers would felt right at home with the long range marches and covering unimproved terrain as well as handling shelter and meeting their basic needs.
If you really dive into the history of special forces and the backgrounds of the original trainers/leaders, you'll see men with well rounded knowledge of ancient tactics/warfare as well as knowledge/training in more modern, yet primitive, tactics like guerrilla warfare and taking on much larger forces (again, something that there are great examples of in ancient warfare like the Battle of Marathon, Battle of Julu, Battle of Badr) as well as ancient units/groups (Medjay, Akkadian, Spartan, Sacred Band of Thebes, Dacian, Varangian Guard etc).
Not too quibble too much, but US Civil War soldiers were marching all over the eastern United States, not just for parades and pageants. They went through rough terrain, with no guarantee of supplies, and many soldiers ended up barefoot when their shoes fell apart. And many times they had to keep it up for days on end in order to escape or catch up to an enemy.
Fair, but they also carried considerably less weight at 40-50lbs versus the 80lbs that a Roman legionnaire carried and the 100-120lbs troops deployed in Afghanistan could be carrying.
No end of scenarios where helicopter / jeep isn't going to work, or are being shot up, or can't fly. Yomping 50 miles across rough terrain, over 3 days with 100lbs and capability to fight when they arrive probably isn't a capability you want to scale back. It's used too often.
There's good reason it's a usual part of the combat fitness test for recruits.
