Is there any country that USA can’t take over or defeat nowadays? 
Afghanistan?
That depends on the conditions you set. If you want to throw morality to the wind the US could probably cream anybody one-on-one (even disallowing nukes). If you want to avoid, say, millions of South Koreans dying in the crossfire or the death of countless innocents then there’s clearly quite a few that fighting would be in issue.
ETA: Though that’s abstracting away the very real but somewhat unpredictable possibilities of countries allying against us, our current allies turning against us (either due to moral disgust, necessity, or whatever other reason), or simply other enemies taking opportunistic advantage of our resources being focused elsewhere.
If you laid those out as separate criteria, I think you’d have several in the “can’t take over” column, and none in the “can’t defeat” column.
Can’t in terms of military might? Probably not.
Can’t because the political, economic, or social ramifications would be unacceptable? I’d say most countries.
The US could probably bomb Haiti to oblivion in 24 hours. Such a thing would almost certainly not happen because the US would be seen as this giant bully and other countries would impose sanctions on us and there would be massive peace protests in the streets of major cities.
We could not conquer China without using nuclear weapons.
You’re confusing the attrition from a hostile occupation facing an insurgency with defeating and taking over.
Part of the problem with Iraq and Afghanistan is that hardly anyone wants or wanted to call our actions for what they were: a swift defeat of the existing military forces and governments, followed by a very long, largely hostile occupation. Occupying a country is different from defeating it in a war.
ETA: There’s no opposing military force the U.S. could not defeat. The cost of that victory would be horrendous in some cases, and trying to occupy some countries would be hopeless.
Yeah. What’s the scenario? The US vs one other country in a one-on-one “fair fight” with no allies jumping in?
Switzerland. Nobody can take over that country because they have really cool knives.
What do you mean by “can’t”? If it means “the negatives outweigh the positives” or “there’s too much downside”, the answer is “All of them”. Or else we would have done it by now.
Of course, by “can’t”, if you mean that it is physically impossible, the answer is “none of them”. We could nuke every country on earth into the stone age, if not the primordial soup, if we so wanted.
Russia and China.
With nukes you get mutual assured destruction.
Without nukes the U.S. simply can’t get enough soldiers and tanks over the ocean to defeat the local forces. We could bomb the crap out of their infrastructure, but we would still be overwhelmed trying to advance and hold territory.
Add the UK and France to this. How many cities would the US be willing to trade for control of those countries?
In a purely conventional engagement, well, who knows. US vs EU wouldn’t be a lot of fun for anyone.
I suspect any overseas country that can reach us with nuclear weapons could do us enough damage that the resulting economic collapse would render us incapable of sustaining our military at anything near its present abilities.
Which leads to another problem; America is very trade dependent. If it started attacking some place that panicked our trade partners, the loss of trade would cause a huge economic collapse and we couldn’t fund our exceedingly expensive military anymore.
There absolutely no other armed forces that could not be defeated in the field by the US military. The size of the US defense spendingcompared to other countries would ensure that.
However there are plenty of countries where logistics of actually facing them in the field (let alone actually launching a fully fledged land invasion) would make it all but impossible. I can’t see how US could ever bring its entire military might to bear on China (invading through Afghanistan and Korea would be their best bet, not the easiest route), no matter how superior their military is.
Not to mention that to attack nuclear powers they would have to accept the fact that most America cities would be wiped out by ICBMs.
These countries would end America’s relevance for the near future:
Russia
United Kingdom
France
China
India
Pakistan
Israel
America could probably take on North Korea if they really wanted since NK doesn’t have very advanced delivery methods, but it’d be a bad idea unless China gave the go ahead or looked the other way. It’d still be a bad idea, but we wouldn’t have to worry about their big brother coming to save the day. Maybe they could smuggle a nuke onto the West coast or into Seoul.
Attacking the major NATO nations would be a really bad idea unless we could convince the others to go along with it.
Yes, but with nukes off the table, the United States could defeat either one.
My point was that Russia and China can’t be conquered either way. With nukes, they nuke us back. Without nukes they’re just too big and populous for us to defeat with conventional weapons, even if we had complete air superiority.
The US maintains and stockpiles, the UKs trident nukes so I am sure if it came to it they could make sure that shockingly the ones supplied to the British subs would be rather unreliable.
Neither India, Pakistan, or I think Israel could reach the US mainland with nukes so I would still definitely say their military could be defeated in the field by the US.
And, of course, so could the others if whatever hypothetical reason for the attack meant the US was prepared to endure some/all their cities being wiped out.
We’re watching you Canada…
Define “defeat”—do you mean “destroy,” “destroy the military capability of,” or “conquer and annex”?
Exactly. Bombing the shit out of an area is a hell of a lot different than occupying it and making it safely habitable.