Strategic nuclear weapons - the original and most widespread nuclear weapons - cannot be used with restraint.

They can. One of the problems that America had, going into the 80s, was that its ICBM force was becoming vulnerable to a potential surprise attack by the CCCP. This concerned them because only the ICBM force, at the time, had the sort of accuracy necessary for taking out hardened targets in a limited strike - like their opponent's strategic forces. And they were understandably reluctant to rely on systems that could only be used for city busting - i.e. the submarine force.

If you're interested in this, I suggest the - contemporary with that problem - documentary First Strike.