The area I am looking at is DSIII's Irrithyl, focusing on Anor Londo, the final area and castle where you fight Aldrich.

We should first breakdown the evironment, since Anor Londo is elevated it's right in the peak of the cold. The cold has inflicted a lot of damage on infrastucture of all the buildings as seen by the cracks in the flooring and the snow that blows in through the smashed windows leaving a layer of thick snow in each of the buildings. Already the US army has lost against the harsh cold climate but lets give them the benefit of the doubt, let's say for story's sake we give them one rime blue frost clump each, just one so that's 1 minute of frost resitance. I do not dictate how the army will work with this. My plan would be to single out the best soldier and to give them all the frost clumps and leave all the others to die. Also guns will jam in that cold a climate there is no way the weapons will survive in this area. Becuase the area is so fucking cold, the soldiers will keep resting at the bonfire unaware that it respawns all the enemies in the area so they'll keep resting and killing and resting, they're farming without a purpose.

Hostile envrionment. If you're a soldier out in the field, it's pretty stupid work. You're given simple direct tasks it's almost too linear, go here shoot there leave a mess, repeat. Tedious monotinous work, much like the enemies whose sole purpose in life is to kill the ashen one. There are so many varieties of enemies in this game, very tough enemies that take copious ammounts of damage to kill. If the entirety of the army wasn't wiped out immediately by cold temperatures then they will almost all die to Sulyvhan's beast, a total of 3 are in this area. Good luck wiping them out. If the soldiers make it to the barrier, uh oh, you didn't kill the deacons of the deep yet? Welp, gotta go back to the Cathedral of the deep now and kill all the enemies there just to acquire one doll that lets you into Irrithyl. The army is really stupid so they'll most likely attempt to blow the magic wall up but it's magic that shit's not coming down without the doll. LET'S SAY they found the doll, they wouldn't but let's say they got through the barrier. You have invisible mobs that attack you out of thin air, you have these 7 - 10 foot tall fire maidens that use their staff to teleport flames and use it as a kind of flame thrower. There's similar types to these which just duel wield swords and have a homing attack which is hard to avoid if you don't know how to roll and the US army doesn't because they don't teach you how to roll. House centipede like creatures that grab you from the waters. Deacons make it back to Anor Londo just to demoralize the army if they decided to go back and aquire the doll from the deacons, the few deacons that remain in anor londo will activate the ptsd in a lot of the soldiers rendering them useless. Oh yeah, the deacons have homing fire balls, good luck with that, not to mention the various silver knights that shoot 5 foot arrows that deal a lot of damage.

Bosses: This area is so hard with the bosses because Sulyvhan creates a clone of themselve's at half health and the attacks almost never end. It's a flurry of consistently punishing attacks that inflict fire and magic damage. Also the pontiff duel wields 2 x 10 feet long swords that explode on impact, pontiff can levitate in the air for 2-3 seconds before diving down and leaving an explosion where they land. If anyone finished off Pontiff then it's not over. You still have the 2 pontiff beasts which are located in a hidden area so the army will most likely miss this. But the area leading up to this part is surrounded by more deacons and 2 giants that didn't die yet. The main boss is aldrich, it's over by this point but come on, it adds insult to injury, the flurry of arrows that rains down following the player almost never ending. More homing magic misslies and tinier versions of these that act like bullets.

Conclusion:

It's safe to assume the us army wouldn't be able to succeed in their mission, if any soldiers were to survive they would all have some sort of severe mental trauma from these events. And this is just one area in the game not to mention all the other areas with lots of variety from drastic changes in the environment to more hostile enemies. The game is not suited to an army that doesn't even have adequate gear to survive in the cold.