You really think that Russia could nuke NYC and it wouldn't invite a retaliation? It's trivially easy to identify it as Russia - the only other countries that could do it are Britain & France (our allies) and China (who has no motive as of yet, and can just barely reach NYC with land-based ICBMs). There's no way that Trump is would let a nuclear attack on a U.S. city go unpunished, and so his very next step would be to nuke Moscow. From there, the countries have a showdown: either they continue retaliating and that's the end of life on earth as we know it, or they let it stop at one city and both countries have their most populous city reduced to rubble.

Both countries know this, which is why neither Trump or Putin are going to launch the missiles.