Faerun would beat the US by virtue of it's Epic Wizards. The US (and Earth as a whole) can't really do much against the likes of Larloch/Szass Tam/Ioulaum. Wizards of that level can comfortably conquer our world without us being able to do anything.
Larloch wouldn't give a damn either way. He's never had any inclination towards world domination. The closest he's come to that since the fall of Netheril was trying to take over Mystra's place in the pantheon. Assuming he survived the attempt, he'd go back to his magical research. Larloch employs people who strive towards world domination.
Ioulaum is also not invested in any of these powers, as far as I can see.
That just leaves Szass Tam, who is the only one that is politically motivated at all. Last we saw of him, he had gone from power politics to trying to rewrite reality. Since attempting the same thing in RL Earth would probably end with Szass Tam getting a visit from the Archangel Gabriel, that's probably not going to happen.
Power politics is another situation. Szass Tam cannot win a direct war against the United States. This isn't an issue of whether or not they can kill Szass Tam, but rather that the US could cripple every city in his realm. Without the economic and military might of Thay, Szass would not be able to continue his expensive magical studies, experiments, and neither would the wizards beneath him. That in turn would leave Szass Tam exposed to local mages who would be more interested in the downfall of Szass Tam than the downfall of the United States.
I also hate to be that guy, but Wraiths. The Wraithpocalypse is a meme for a reason. Drop a single Wraith in a highly populated city and you'll have millions of wraiths in short order.
You actually won't.
- A wraith has no power in natural sunlight. So such reproduction cannot simply spread without friction.
- Wraiths shun areas of bright light. Somehow, I don't think dropping a wraith in NYC will cause the entire city to fall. It's constantly bright and full of light. You'd need a considerably powerful wizard to compel the wraith to attack.
- Wraiths tend to haunt areas of darkness and death.
You would most definitively have a boom in a wraith population in NYC, but they're not going to overrun the city. In fact, you might actually make NYC safer, since most of the people the wraiths are going to catch are going to be criminals, bums, mentally unhinged, or troublemakers.
You can't kill them without magic and their numbers can grow exponentially. Every victim they kill becomes another Wraith.
Also incorrect.
- 5e equated radiant damage with lasers. Since the US can produce lasers (albeit, not as efficient battlefield weapons--but still possible), they're not impossible to kill.
- 5e made them resistant to damage, not immune. So enough bullets will still kill one.
- 5e also only made them resistant to fire, not immune. So flam throwers.
- Ignoring 5e altogether, they can still be harmed by silver weapons.
What you will generate is an entirely new class of economics as wraith hunters employ new weapons and gear to combat the monsters. Flash grenades, floodlights, silver spears/crossbows/bullets, and possibly lasers (or flamethrowers) to destroy them.
Only if they are very young. As Asoiaf dragons grow, their scales harden until they are all but impenetrable. The Dornish got unbelievably lucky with Meraxes. They did a 1 in a million impossible ballista shot to the brain through the eye. On adult dragons, the eyes are their only weak spot.
When the Dornish tried to repeat their miracle against Vhagar and Caraxes, it ended with all of them dying. They put as many ballistas as they could on every ship they had and all they managed was to get their fleet destroyed. Aemon and Baelon were celebrated for their bloodless victory (nobody on their side died).
Of course, rockets should be able to kill dragons pretty easily, but ballistas are just not very effective. It's almost impossible to kill an Asoiaf dragon with in setting technology. The best counter for dragons is having your own dragon. Otherwise, you are probably just going to get burned alive.
Ballistas are basically useless, but I doubt anyone in the US is going to use anything less sophisticated than WWII weaponry in a siege battle with a dragon. And most likely they're going to be taken out by a jet, a high power cannon, or AA weapons. The biggest issue with dragons, is that they can perform spells, have long lives, and can essentially melt away into the population.
*finds the oldest thread in the Versus Forum*
They do have the Tarrasque but as for Gods... it's not as big a deal if you try and reconcile the fact that they'd only be able to have their 'Avatars' appear as errr avatars of their Gods instead of engaging in some manner of divine fuckery to instapwn the Modern World.
Also thanks to 5e we don't need that sort of +1/nonmagical weapon immunity with many monsters. It's instead reduced to various resistances so eventually even those Great Wyrms might get dragged down by superior modern day firepower.
This really breaks down to what sort of D&D you're referring to. Lore is often different than game mechanics, which is what 5e is strongly associated with. Not as much as say, 4th, but the monsters are mostly just weapons and obstacles. 3e and prior versions treated them as living creatures, so you could more easily discern how they would integrate in real life. That said, we know from lore that nothing short of lower end gods could really do anything against the US in a direct war. Even a fairly older dragon is not going to be able to survive a fight with F-16s without copious amounts of magic being used.
The only thing that could fight them would be a lesser power, which would be capable of essentially releasing magical nukes.
Of course, most high powered magical beings are not going to get in a fight they can't win and will be better off working through the US's own system and with indirect warfare than head-on.