Listen my scenario
American guy, who sit behind big red button is pissed off, his girlfriend left him with handsome Chinese guy, and he push the button, China response,
Russian button guy is drunk, to him it look like the US nukes are for Russia and he fire back, but he is so drunk, he push wrong button and sent nukes to Korea.
Korea is not ready yet, they even not refuel rockets, but they most to respond,fire Nukes at Russia, but, but, but ..... oh no, there is no enough fuel to reach Russia, it drops in sea, big boom, and BIG Tsunami as consequence,
Japan, Indonesia, Australia, India and all Oceania are under watter,
Of course, US survive without any victims, like in movie, and of course
blame "Iran nuclear program" and blow it up. African tribes eat each other,
Since all oil deposits and rigs are destroyed, (Middle east Russia and etc)
US are in energy crisis and in next few hours every US citizen dies.
New South America alliance go in war with Canada for US territory
and EU people dies of boring.
In the end only Eskimos and penguins left to fight for future world domination.
No, there will no be WW3, at least not for some 300 years !!!