It should be pointed out that the US does not have a nuclear arsenal capable of destroying even our species, let alone all life on Earth. Even the combined nuclear arsenals of the entire world aren't enough to wipe everyone out, humanity would survive. Fallout is extremely nasty, but it's not planet killing (at least not the amount humanity is currently able to produce). An interesting thing I read in a US nuclear survival thing, the amount of cancer caused by a nuclear exchange between the US and USSR in the 80s would be less than the amount of cancer prevented if wearing hats outdoors became a worldwide fashion.
Now, the death toll would be appalling, you'd fuck civilisation right up for a few centuries and all. That's what they mean by "end of the world", it really needs "as we know it" unless you are being pedantic.
In my experience, the effects of nuclear war are exagerated, possibly to change people's attitudes against it...presumably there are those who don't find massive death tolls, loss of infrastructure and governmental collapse to be scary enough.
On the other hand, the common or garden problems are overlooked, they don't seem exciting an nuclear enough. Lots of people will be horribly maimed by flying glass, and the medical facilities left intact will be swamped. You'll lose power and running water, you've got cities full of decaying corpses, your transport system is wrecked etc.
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But, if you had a planet killing thingy to use, or not use...dunno. One of the things a UK PM did/does when they enter office is give sealed instructions to submarine captains to be opened in the event of a nuclear war with the USSR (possibly they still do for Russia nowdays), whether they should launch their missiles in retaliation or not. That's one hell of a decisions to make.
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Actually, given that this question (or something very much liked it) was such a big deal for UK PMs, this is one of the more interesting (and unlikely to end in a shitstorm) questions I've seen asked here.
