The problem with the US is they will go in all guns blazing with sanctions and make negotiation all the more impossible, which is why I'm really sorry that the UK have taken the lead over sanctions, because whatever we hit them with, you can be sure the US will want to trump that.
Putin isn't going to back down, if he's showing an unhealthy nostalgia and bitterness over the breakdown of the USSR, he's not going to allow anything to be resolved without years of underhanded and probably anonymous responses to key figures. If he can't hit back financially, he'll do what Russia have always done.
A lot depends on China and Germany now, weighing up where their loyalties lie moving forward. Germany have a lot to lose if they side with Russia, given the historical connotations and the relationships they have built with the west over the last 70 years.