If US would attack, UK and some other western countries would back them, so by US it should be US+EU attack Russia, basically WW3 is on.
The United Kingdom might back the US, but other than that, I am not sure about the other members. Germany and France are likely to stay out of the conflict. The same seems to be the case with Spain, Greece, Hungary, Italy, Portugal, Czechia, and Austria. The smaller Russophobe nations like Poland, Lithuania and Estonia might join the war against Russia, but they are likely to suffer disproportinate levels of damage in a counter-attack from Russia.