The Trump administration's Nuclear Posture Review retains a commitment, first made by Obama in 2010, not to "use or threaten to use nuclear weapons against non-nuclear weapons states that are party to the NPT [Non-Proliferation Treaty] and in compliance with their nuclear nonproliferation obligations." Today, all 184 non-nuclear weapons states under the treaty meet that criterion.

Trump's reaffirmation of this pledge, known as a negative security assurance, is significant. It rules out U.S. nuclear retaliation against these countries, even if they were to use chemical, biological, cyber or other weapons against the U.S. or its allies.

Much of the commentary on Trump's NPR has focused on how new, low-yield nuclear weapons and a focus on non-nuclear strategic attacks might lower the threshold for nuclear use. These concerns are justified. But they apply to a very narrow set of circumstances involving countries that already possess nuclear weapons, which have long been the focus of our deterrence strategies.

In U.S. strategy, the fundamental role of nuclear weapons is to deter their use by others. For all the shifts in the new NPR, this role remains unchanged.

Ivo Daalder is the president of the Chicago Council on Global Affairs.