It has never been formally stated, at least not in a diplomatic ceremony or treaty, but well before the end of the Cold War a focal point norm had gradually developed among the nations that possessed nuclear weapons that nobody would be the first to use nuclear weapons, although there was a sort of implicit remnant of "Unless we are seriously invaded and our national existence is threatened." But the very lack of statement in any official way was part of what made the focal point norm exist. Many credit Thomas Schelling with being the crucial figure in all this, both by inventing the idea of focal points in game theory (which was what he officially received his Nobel Prize for) as well as for privately and behind the scenes pushing specifically for such a focal point norm, with apparently getting at least the relevant US figures to go along with it, with the US of course being the only nation ever to actually use nuclear weapons against another nation (and I am especially aware of this having just been at a conference in Tokyo). And it was US figures, most notoriously the late Gen. Curtis E. LeMay, who most loudly advocated using them in various situations, such as in his old "Bomb them back to the stone age," with regard to North Vietnam.
Now Donald Trump has blown apart the focal point, even if few seem to be focusing on that as perhaps the worst thing he has done so far in his generally nauseating campaign, where there seems to be more attention on how his wife has behaved and what size his body parts are. He has allowed as how he might consider using tactical nukes against ISIS, and it would appear that Ted Cruz does not disagree with him, although maybe the Dem candidates do, or at least Bernie. But the cat is now out of the bag. No longer is use of nuclear weapons not in response to a nuclear attack a forbidden norm/focal point. Anything can happen.
OK OK, I recognize that at least one commentator in Russia made noises about using nukes against the US, and now Kim of North Korea has produced a video of a nuclear attack on Washington. But neither of these were or are really taken seriously, at least not in the near future. But Trump is another matter, and if he gets into the White House with his finger on the nuclear trigger, all bets will be off. All that effort by Thomas Schelling will be down the drain.