Most of people, including many Americans, believe that US honor is quite limited. Many believe that the USA will betray Europe (or Europe will betray America) for some money (even without actual threat for human life) to half of US population (with most popular point of view about 20 mln killed). Whatever are your obligations and if Russians can kill more than 20 mln Americans and right now you can make a reliable peace with Russia, well in this situation America wouldn't attack Russia first, whatever Russia do with Europeans (including nuclear strikes).