No leader of a nuclear state is going to initiate a first strike against another nuclear state with a counterstrike capability.
That is the generally accepted strategy and logic with which I agree.
Moreover, I assess Putin to be a Russian Nationalist that does love Russia and Russians, as authoritarian as he might be, so it's not him I worry about.
I assess Obama and most around him to not love America at all, and themselves above all.
They are true believers.
Most pols are narcissists. As such, they have highly-developed instincts of self-preservation. There is no chance Obama will initiate a nuclear strike against Russia, mainly because it would be bad for him personally, meaning there's a substantial chance he would be killed along with all of his kin. Like many pols, Obama is a true believer - in what is good for Obama. And there is no scenario in which a US first strike followed by a Russian counterstrike that kills him and everyone he knows is good for Obama.