So Heres how im gonna Explain all of this

ill break into a few parts

-Advantages and Disadvantages

-Biggest threats on each side

-and How i think the battle would go

Now For The Advantages and Disadvantages

Lets Start With union

now for this im only using the main three verses used in all unions

Highschool Dxd

Mha

And Rwby

and the reason why is because there always present in every verse and that since union stories have All universes have different universes in them like Fairy tail and dragonball so this is the most fair way that i can make this

So back to Unions Advantages and Disadvantages:

Advantadges:

-the fact they all have powers and superhuman physiology

-They do have strength in numbers i believe due to the devils in hell (though that might not be as much as most might think since the devil population plummeted after the great war so it might not be as severe)

-Now they have interesting weapons thanks to Atlas and their use of dust weapons and the paladins

Now for Disadvantages

Lack Of Experience:  and what i mean by that is they do not have the experience in fighting in modern wars. and what i mean by that is no union ive seen that have fought in any wars since the great war and the fact they only have ever really fought salem and the grimm and AFO And his villain group

Lack Of Aircraft technology: Lets be perfectly Honest here outside of atlas's ships they really dont compare to what the usa has

I mean come on what is a atlas manta gonna do When a F-22 Raptor shoots it out of the sky from beyond visual range?

-Lack of Intelligence Gathering capabilities

Given how well ozpin set up the communication network in remnant... i think we can all agree was flawed as hell

and i could keep going on but lets move on to the USA Advantages and Disadvantages

USA Advantages:

Better trained and equipped soldiers

Far more firepower

Has Better air force by a long shot

Has better intelligence gathering And espionage technology and knowledge

Has far more experience fighting modern wars

Has a insane military budget

Disadvantages:

-wouldnt have the ability to harm the supernatural

Now for the biggest threats on unions side

-issei: Given his sacred gear he is a formidable and dangerous Threat Though he isnt the brightest at times

-Deku: Given his quirk and his intelligence he will be one of the bigger threats the Usa would have to deal with First

-All Might: No big surprise given his feats but since this isnt prime all might he cant stay in his hero form long but hes still strong enough to do massive damage

Bakugo: while Dangerous and powerful with his quirk he isnt always the brightest and can probably be outplayed

Todoroki: Pretty self explanatory am i right?

Sirzechs: The Leader of hell and the current lucifer and he does have a massive amount of power

Now for the usa

... literally the whole army is a threat like come on

Now for how this battle would go

So The way i see the reason war is starting would be that the USA and the world governments had enough of union failing to stop the villains and grimm fully and they would of gotten intelligence of their corruption and their extensive crimes against humanity (aka: their lack of care for civilian lives and collateral damage 

And even before the usa would declare war they would spend months if not years before hand gathering intelligence on unions capabilities and would do training and preparing for all the strikes against all of unions military structures and prepare special forces to take out the high priority targets in union

And yes they would develop The technology to be able to hurt the supernatural thats why they always prepare in advance to make sure they have the best possible chance

now for the wars start

Around 5:00 Am  

The president of the Usa goes on national television and States to the nation and the world that the usa was declaring war on union to not only destroy unions influence on the world but it would imprison the headmasters and the students who have caused nothing but destruction and international terror and would make sure they have no real power Ever again. and after unions dissolvent they would take charge of dealing with the villians.

After the Broadcast Union members would be abruptly woken up by the sounds of multiple massive explosions as the usas F-22s fired missiles at unions anti-air defense's to make way for their  strategic bombers (B-52s, B-2s, B-21S,) Started to lay waste to unions military infrastructure.

union would scramble to put out bullheads and forces that can fly to intercept the bombers but they would be intercepted by not only more F-22s but F-16s, AC-130s, F-35s and A-10 warthogs would intercept them and union within the first few minutes union would experience massive losses of aircraft and alot of heros and supernatural's would be lost. Though while the usa takes heavy losses to their air fleet but unions ability to defend themselves from more air attacks and their infrastructure would be Beyond devastated.

While union would be recovering from the air attack US ground forces would be thundering towards union with a armored spear head lead by the feared M1A2 Abrams while being supported by the also feared Apache attack copters and M2 Bradley's along with several thousand Soldiers .

Caught by surprise by the speed of the USAs advance Union would be able to set up some defense's but  union wouldnt be ready for the apaches to blow right through their defense's and a massive battle would ensue and both sides would sustain losses at a rapid rate but it would be a slow but easy victory as huntsmens, devils, and "heros" would be decimated and or imprisoned as union soldiers gets basically massacred by US Forces

after several days union has nearly fallen with only sirzechs, Ozpin, deku, bakugo, Allmight, and rias perrage (minus asia, xenovia and irina at this point) would be on the backropes till they are eventually captured by US Forces and taken into custody to be tried by international Courts for their crimes which would go as well as you might expect >:)

AND END OF SCENARIO

So thats how i think the Overall conflict would go but ill make some points here incase some of you call me biased towards Irl Usa

1: union has no real Military doctrine and any they do they would only have it to deal with the grimm and villains.

2: Their not prepared to fight a actual modern war given how they probably usually have international protection because of their powers and whatnot

3: union is usually just a really big city/academy so the entire US air force has a rather easy time bombing it into absolute oblivion without anti air defense's

4: While it would take a few days for The USA Forces to take over  union city the usa is extremely experienced in Urban warfare 

5: alot of the heros and supernatural arent fully bullet proof especially the rwby hunters/huntresses and the Most of the main heros from MHA

I could go on and on but i would be here all day

if you all wanna tell me how you think this battle would go let me know in the comments

but for now Peace everyone! and have a good day