People don't understand what a nuclear strike really means. First, if someone was intent on nuking the United States, they could. China has ballistic capabilities that are an effective work around tracking and targeting. Second, a "nuke" isn't as much an issue as other types of bombs that can cause massive amounts of destruction and are easier to deliver.
...and no, the jet stream carrying fallout isn't an issue. If it were, the nuclear testing that's gone on for decades would have made EVERYTHING horribly bad.
The big question comes from the financial sector. A detonation of a nuke or H-bomb would send the markets all over the world straight to hell in a matter of hours. North Korea would go into economic desolation, China would suffer massive public panic, other Asian countries would blow their **** and production would be somewhere between nowhere and nothing. Western economies would capitulate, crash and whatever was left when their government's responded, likely confiscated, frozen or regulated with the same effect. Russia would go full retard and impose government control on every aspect of life. The effects would be global and severe.
If it became a war, it would be a global collapse of every aspect of modern civilization. That includes modern ethics and morality.
...With so much of Russia's economy tied into western markets through SWIFT, I wouldn't be suprised if Russia turned a crisis into an apocalypse.