The wider role for nuclear weapons against “non-nuclear strategic attacks” was one of several ways in which Trump’s approach differed from his predecessor. The threat of climate change went unmentioned. The word “climate” was only used four times in the National Security Strategy (NSS), and three of those mentions referred to the business environment. Americans were instead urged to “embrace energy dominance”.
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The NSS policy document criticises the downgrading of the role of nuclear weapons in the US security strategy by previous administrations since the Cold War, and suggested it had not prevented nuclear-armed adversaries expanding their arsenals and delivery systems.

“While nuclear deterrence strategies cannot prevent all conflict, they are essential to prevent nuclear attack, non-nuclear strategic attacks, and large-scale conventional aggression,” the NSS said.

“Non-nuclear strategic attacks” represents a new category of threat that US nuclear weapons could be used to counter, and points towards likely changes in the Nuclear Posture Review expected in the next few weeks.

https://www.theguardian.com/us-news/2017/dec/18/nuclear-weapons-trump-national-security-strategy