There are two types of strategic nuclear targets. The first is counterforce, which is military and leadership targets. The second is countervalue, which is a nice way of saying cities and other civilian targets. The threshold for a strategic nuclear attack is obviously much higher than for a tactical nuclear attack, which already has a rather high threshold.
I think a tactical nuclear strike is far more likely to occur than something strategic level, and things would have to be very grave indeed to let the nuclear genie out of its bottle again even for tactical use. In some ways, tactical use is even more dangerous, because it risks becoming a norm in warfare, and thus nuclear attacks in general risk becoming a norm.
Then you have to consider that a terrorist group can't really be deterred by nuclear weapons because they don't care, and that the average person has nothing to do with the decisions that the state leadership makes, and you can't even really make a case for nuclear strikes in retaliation for chemical and/or biological attacks. Assuming that the military and political leaders of a nation want to minimize casualties on both sides, I think they will likely resort to tactical nuclear strikes against military targets, ideally any forces that deployed or have the potential to deploy chemical and/or biological attacks.
If you're really looking at a strategic level attack, I could see a nuclear power executing such a strike to prevent a non-nuclear power from crossing the nuclear threshold. It would be an extreme action, but if a country is considered dangerous or radical enough and the site or sites are in a remote area to minimize civilian casualties and fallout, it might be more politically acceptable than otherwise.
I should also point out that there is the concept of the nuclear taboo, which says that the threshold for nuclear weapons use (and thus the likelihood of use) has continued to increase every year that there hasn't been a nuclear attack. I think that prior to 1970, the nuclear taboo might be loose enough to make a nuclear strike a real option. It would be only 25 years into the nuclear era, and the Nuclear Non-Proliferation Treaty won't have entered into force yet. It's going to be rather difficult to maintain the NNPT if nuclear powers are using nuclear weapons against non-nuclear states.