It goes without saying that I am not an expert, most of this is just me poorly retelling what I've heard from others.
The US is currently attempting to reindustrialize in order to increase their chances of success in a great power conflict, not to mention that they are currently engaged in a naval arms race with China, so the chances of a any war happening between the US and either China or Russia is likely.
If the US goes to war with China, they will do a decent job early on but its likely that they will lose in the long run. They do not currently have the productive capacity or population to win a long war against China. There's a good chance that a lot of countries will be involved, it might genuinely become WWIII. As long as the US retains its current regime, the chances of such a war happening as time goes on is high. There's also a good chance that the US will use nuclear weapons.
If the US goes to war with Russia, the US will probably win so long as they can drum up enough support from the EU, which is likely. The obvious scenario by which this happens is an escalation of the Ukraine conflict, which I'd flip a coin on since I don't think either side will just win at this point. I think Russia is more likely to favor a negotiated settlement that lets them keep Crimea and the Russian-speaking majority regions of western Ukraine, whereas the US will prefer it becomes a perpetually frozen conflict like what's happening in northeastern Syria. China will probably intervene in some way. Low but non-zero chance of either the US or Russia using nuclear weapons, though the US is far more likely to do so since they are the only country so far to have used nuclear weapons in war.
If the US goes to war with Iran, it's very likely that they will win. Iran has too many regional competitors and the current regime is both economically crippled and very unpopular with the general public, a draft will likely result in mutinies. Russia will probably intervene, at the very least to provide materiel.
If the US goes to war with North Korea, its almost inevitable that they will drag China into the conflict.
It's very unlikely but the US might annex Cuba or invade Venezuela, in which case they will win with almost no effort. A single aircraft carrier and some marines would be enough.