"Nor is the US."
Translation:Les États-Unis ne le sont pas non plus.
We would not say that. Remember that this is a very short sentence, like a brief comment after someone said something containing verb be/être:
"Russia is not at war. // Nor is the US."
"La Russie n'est pas en guerre. // Les Etats-Unis non plus (or longer: les Etats-Unis ne le sont pas non plus)."
This sentence is a piece of conversation which, as usual, has no context, so it is up to you to imagine it. Example:
A: La France n'est pas une dictature. B: Les Etats-Unis ne le sont pas non plus
"le" stands for "une dictature", to avoid repetition and still give a hint as to what the US "is not".
"non plus" translates "nor" or "not either".
By the way, the French are much more strict on singular/plural than English speakers. The US being a plural, the French will never use 3rd person singular to refer to "them".