China - everyone knows this; lots of front-line weapons and HUGE numbers of older ones. The saying goes that there aren't enough missiles bullets in the US arsenal to shoot down every Chinese aircraft. The US would take massive losses in the air against the PLAAF. Naturally we would deal them a savage blow, too...but in the most likely scenario (the defense of Taiwan), we are fighting on their home field.
North Korea - THE most heavily defended place on earth. Over 70,000 pieces of anti-aircraft artillery in a country the size of Mississippi. Has been preparing for war for 50 years...has underground airfields, missiles that pop up on elevators on mountaintops, and a ton of MiGs. Most importantly, they have a population and military that is fanatically motivated -- very similar to the Japanese in the 40s. If NK ever decides to roll south, it will be a VERY, very bloody war on both sides. Same story as above with the home field advantage.
India - Much smarter and technologically advanced than we give them credit for. The Cope India exercises showed that they have not only learned from past mistakes, but come up with a few tricks of their own. Don't be lulled into thinking that India would never be a US foe. Allegiances in south Asia rise and fall with the winds...and this alliance depends heavily on how much the US sides with Pakistan.
Russia - Athough the Georgia war exposed some problems with their air force (it has atrophied significantly since the cold war), this is the country that designed and built all those aircraft and missiles that equal or better US equipment.
Some of the ex-Soviet client states, like Ukraine and Belarus fall into a similar category with respect to their equipment.
Iran almost fits in there, except their air defenses are what they do well...not their aircraft.
The problem with your logic is though all of these states have decent Air Forces, or air defense capabilities, none of these countries have a quarrel with us.
China: We owe them too much money for them to want to beat it out of us, they can milk us dry for the next century.
North Korea: Imploding slowly from the inside, you can have guns or butter, not both, I sincerely doubt we'll ever fight them. That being said, this is a fairly legitimate threat. However, the threat comes not from unfriendly PRK aircraft, but rather from a million plus armed Koreans overrunning the DMZ
India: The eastern slice of "who-we-can-trust" bread in a middle east sandwich. As long as we're fighting over there, and helping quell the rise of Islamism, I doubt very seriously that the worlds largest democracy will want to quarrel. Beyond that, you remember from political science class, "no two countries with McDonalds restaurants have ever attacked eachother?" (that may have changed, but there's something to be said for fairly affluent democracies not wanting to slap eachother around).
Russia: The Russians will screw with us (e.g. TU-95s, and Venezuela bases) but if we didn't go to war during a 50 year nuclear standoff, then you can bet your ass that the Russians, as smart as they are, are not going to fight with us right now. Especially not when the economy is in the toilet. They couldn't afford a real war if they wanted to, look at how little effort they've put into the conflict in Chechnya.
Ukraine/Belarus: Ehh, I've been to Ukraine, and frankly, though they have some decent equipment, I highly doubt even if they decided to fight that they would stand much of a chance. Really, all of the CNG (CIS) states' militrary forces are in somewhat of a state of decay and disrepair. Just look at what happened in Georgia.
The countries I'm worried about are Pakistan (they've got Nukes), Iran (think straights of Hormuz) though they wouldn't put up much of a fight, it'd still be bad for the economy, and finally, Mexico. Its crazy close to home, and unstable as hell right now, plus we get a hell of a lot of Oil from there. And all three of those wars would be anti-partisan actions, similar to Iraq after the immediate opening days of the conflict, if they happened at all (Pakistan is already kind of going).
Now, I'm going to put something out there. We've already got a super stealthy bomber, and fighter bomber, the B2, and F117 respectively. I'd venture a guess that no matter where we go to war, there's a good chance that any countries planning on using their air forces wouldn't even be able to get them off of the ground, or for those they managed to hide, or if the sheer numbers of aircraft dispatched was too great, the turkey shoot would be on. Again, the F15E has never been shot down (there might be classified cases, but publicly, never). Plus, we have F22s out there already. If they need to dogfight, I doubt there's anything on the planet that can really go toe-to-toe with 187 of them. an additional 113 won't make or break American air superiority. I could be completely off base with this, but I'm a fairly disinterested party in these respects.
