I wonder, "back down" how? I mean, I could imagine Putin perhaps being allowed to keep Crimea for some considerable length of time. As for moving on the rest, I really don't know enough to be completely sure. Though I'd be surprised if NATO set out for more than a carefully announced, limited air campaign. The Baltics may be in, but I don't imagine that there are actually boots on the ground to defend everyone (rather more buffer state treatment?) and then I imagine Ukrainian terrain may not be so easy to defend conventionally. There is some speculation it could turn into something of a Bosnia/Kosovo situation, if you have a real stomach for brinkmanship perhaps.
As it is... The question could be, just how long is Putin really interested in playing the old Soviet "who can or can't really stand alone economically" game? I wonder if he anticipated anyone actually applying sanctions. There is a whole lot of rhetoric about those now.