[quote=So Boerd]Americans, what level of Chinese provocation would warrant a US nuclear strike? How about Russian?
[/quote]

A nuclear strike does not end the world. Nuclear retaliation ends the world. So the only situation in which a nuclear strike is advantageous is one in which there are guarantees that there will be no retaliation. The example I use is, if North Korea were to amass an army and launch a bombardment of Seoul. As long as China gave the okay, we would almost certainly be wise to nuke their army formation, rather than trying to tackle them with conventional forces. 

By the same logic, there aren't a lot of scenarios involving nuclear nations (China, Russia, Pakistan, et al) which would make nuclear weapons a smart option, because the only way to ensure zero-retaliation is to ensure that every nuclear option of the enemy is destroyed. That's not something you want to bet the world on. That's why rogue states want nuclear programs in the first place -- it's only MAD if you've got something to shoot back with.

Edit: The other justification would be, obviously, a nuclear or existential provocation. When the circumstances are such that only a nuclear strike can protect your nation and/or countrymen, the military has an obligation to nuke the living shit out of the threat. Fortunately, there's virtually no danger of such a scenario for the foreseeable future.