system
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human
[Fallout] What did the army of the old world look like?

So, with more weapons than you can shake a stick at, as well as kinetic (bullets), Direct-Energy, and plasma weaponry, it makes me wonder what did the army actually look like back then? Did your standard soldier just lug around a plasma rifle? Direct-Energy? Was it reserved for special troops? How prevalent was power armor? Where they grouped in their own little squads and used like shock troops? What was the makeup of your average squad?

It honestly boggles my mind!
gpt
Interesting question, Nuka-fueled Wastelander speculation incoming.

Plasma were definetively not standard issue. Just look at the things! Either they are hilariously big and bulky like the Winchester P94, or a mess of exposed cables and components like Winchesters more conventional-shaped urban plasma rifle.  
They still are common enough in the wasteland, though, so them being reserved for special occations sounds fair.  

Power armour were developed as a means to easily carry heavy weaponry, essentially turning an infantryman into a bipedal tank.
Power armour were used, at least intended to be used, in seperate squads. loaded up with heavy weapons like miniguns, fat men and other stuff a regular infantryman would have a hard time carrying, they would spearhead attacks like a conventional tracked tank would.

The US military were moving towards making energy weapons standard issue, but it is slow to replace the weapons of an entire army, and frontline units like those serving in China were surely prioritized over the home front, so regular bullet-spitters like the  R91 were commonplace.  
However, the US military were moving towards making the AER9 laser rifle standard issue. Makes sense. Lasers pack a bigger punch than the old assault rifles, and a "point and shoot" interface as opposed to the slow plasma projectiles.  
The AER9 is not top of the line laser weaponry, but the sturdy case and more reliable design are needed to survive the harshest enviroment known to man: the hands of an US grunt.
The Sole Survivors old war footage lend support to this. He and his squad are all equipped with laser rifles.

So, to sum it up, and assuming we are talking frontline armies, not home front/national guard units fighting civillians, an assault would be spearheaded by power armour units lugging around miniguns, rocket launchers, fat men, and probably stuff like plasma rifles if it were deemed advantagous.  
Then comes the rest of the spear, the bulk of the army. Regular grunts equipped with combat armour and likely a laser rifle. Frontline units get priorities on the good stuff, so if you are serving behind the lines, you might have an assault rifle instead.  
Finally, as we see from the Sole Survivors footage, assigning power armour units to regular squads to carry heavy weapons are not completely off the books, but it is hard to know if this was intended to be a permanent replacement for the shmuck lugging around a light machine gun, or just a temporary replacement, to give patrols extra firepower when the power armour units are sitting on their asses anyway.