What would it take for the USSR to approve a US nucleat strike?

Not on Soviet territory, of course.

What instances would have justified a US nuclear response/first strike on a target that even the Soviet Union would have agreed was justified?

The massacre of an entire embassy? A dirty bomb in NYC? 9/11 on multiple American cities?
 
Not on Soviet territory, of course.

What instances would have justified a US nuclear response/first strike on a target that even the Soviet Union would have agreed was justified?

The massacre of an entire embassy? A dirty bomb in NYC? 9/11 on multiple American cities?

A worse 9/11 (substituting Russia for USSR obviously) perhaps with a successful decapitation strike during the State of the Union like the Tom Clancy novel could see condoned use of a nuclear strike on known Al Queda positions in Afgahnistan

Other place I could maybe see it, is Gulf War 91 if Saddam launches full chem/bio barrages/scuds and air strikes on assembling coalition troops or into Saudi Arabia/Israeli cities, which could trigger a tactical nuclear strike(s) against republican guards or iraqi army positions that are away from civilian centers
 
Using nukes on a terrorist organization not really a thing, now say Iran was proven to have given some terrorist organization a nuke which they used on the US in some way, then the US nuking Iranian military sites and their nuke facilities is likely. IMHO the USSR/Russia is not going to make too much noise about the USA responding to a WMD attack with nukes as long as they are not taking out cities - in the Iran case military sites, nuke sites sure, Tehran no (at least not with nukes). The Soviets/Russians are not going to risk putting their male members in the sausage machine defending somebody else who was stupid enough to do something the USA decides to use nukes against.
 
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