Okay it's Hypothetical but the United States is looked at as a World Super Power and therefore, because of our world wide reach, the target of just about everyone else that isn't an ally. In this age and time I think our biggest threat is from China today even though it's Russia that has been more confrontational and has slightly more Nuclear Weapons than we have. Russia is said to have 7300 and the us some 6900+ and China with some 260. Trouble is with Nukes the number isn't that important due to the overall damage one would do.
The reason I said China is that China has become an economic behemoth and is jockeying to be number one. Also China has began building their military up to a point that they want to extend their power worldwide also and be a check to the United States. Before China wasn't such a threat because they never demonstrated any kind of desire to venture outside their own domain but today China has a vast and active Space program and technologically very well might be a match for the United States, according to some. Russia will always be a threat and have aspirations to restore their power to that of the glory days of the USSR but to do so today would put them in direct conflict with NATO and I don't know that they want to chance that.
Some may look at the geopolitical world today and say that nations like Iran or North Korea are our greatest threat because of the tendency that if something happens there it would quickly escalate to involving Russia or China. I just hope that calmer heads will prevail and that there could be someway to enjoy some peacetime for a change. I know that there are Islamic Radicals that would like to have jihad with everyone and anyone they can and are a threat to everyone at any particular time but given the scope of things unless they get their hands on a nuke and a way to get it into the country then chances are they will continue to be an aggravation and a small target for us but not the threat that China or Russia could be.
Any Nuclear conflict is unthinkable regardless of whom it's with. If anyone is curious what would happen in a blast there is a website that allows you to play around with what if's of a Nuclear detonation.
The largest Russian Nuke is said to be 100 Megatons but there is speculation that they have doubled that and have a 200 Megaton weapon in their inventory.
The winner of a Nuclear War is ... NOBODY, Everyone loses!
