The United States started the Great War with units equipped with T-45 Power Armor, amongst everything else and it allowed them to stalemate China in Alaska and invade China proper in 2074. But it's telling and important to note that in 2076 the T-51 Power Armor was introduced to both Alaska and in the Chinese theater and the reversals against China began immediately. After ten years of fighting, China was going to be evicted from Alaska and in the Yangtze Campaign, it was the T-51 Power Armor that allowed the American Units to strike and counter Chinese tanks and infantry formations with superior mobility and firepower as well as cut their supply lines to ribbons AND even capture several of China's major cities and all of that happened in less then a year.
In Fallouts 1 & 2 as well as to a lesser extent Fallout 4 and I'm assuming 76, they treated Power Armor as the literal tanks they should be. Though in the earlier games, the Power Armor suits were stated to have a power supply that lasted a hundred years instead of the constant hunting for power cores to top off the suit.
Power Armored units can shrug off most small arms including those from punier laser and plasma weapons as well as explosives like grenades and the like unless it hits them directly. They are like walking tanks because they can also carry everything from gatling lasers to oversized plasma rifles as well as things like gauss rifles, missile launchers or mini nukes... while also possessing NBC protection.
I'm not sure how modern day forces will be able to counter the use of extensive Power Armor backed forces. I'm sure a tanks main cannon could cripple or kill a Power Armored trooper but you'd still have to hit them and not get slagged by a rocket launcher or other anti-armor weaponry. Same with all sorts of other man-portable armored weaponry. Maybe an ATGM like a TOW or Javelin or NLAW could penetrate the armor but could a direct hit from an RPG-7 or an M72 Law do the same? Or just do light damage?
Then you still have units with the T-45 Power Armor which is still significantly more capable then modern infantry kit. Then you have regular combat armor which in itself offers pretty good protection, comparable if not better than modern infantry body armor.
And then the Fallout USA has laser and plasma rifles which aren't something modern forces face. The commonly issued weapons are the Wattz 2000 Laser Rifle (seen used by a Marine Sniper in Nanjing China in a magazine) and the AER9/12 Laser Rifle which was more of an infantry assault rifle replacement. Then you have the more powerful but perhaps less focused Wincester P94 Plasma Rifle (the big boy gun from the early Fallout games that melted people) and the later games more domesticated Urban Plasma Rifle which was also still pretty powerful and capable of goo reducing people.
And then in addition to all of that you have the aforementioned military bots. Sentry Bots and Mister Gutsies most notably would be quite useful. Eyebots maybe for reconnaissance. And even Assaultrons were apparently in use on frontlines as early as 2072 which... no thank you.
Fallout USA probably also has superior industrial capacity, and population... and a wartime footing as well. What's undefined is what they could bring to bear beyond the infantry level. We know they have artillery, the howitzers seen seem to operate like traditional artillery, whether in Fallout 2 or the later Bethesda games.
There are tanks. We see double barrelled ones in Fallout 4 but there's no real information on them. There are large APC's which, again very little detail. And we see 1950's era inspired jet fighters on board the carrier in the Washington Naval Yard in Fallout 3. They look like P-80's but there are no P-80's with folded wings for carrier use. Also not sure what the Flight Helmets looked like in 1950 but the ones in Fallout 4 don't match up with them I'm assuming.
And you also get things like laser cannons that can somewhat reliably intercept ballistic missiles, energy based forcefields, and orbital weaponry like Archimedes and more menacingly the Bradley-Hercules satellite from Broken Steel.
