Playing today in the theatre of international relations is A Beautiful Mind, Ronnie Howard's 2001 film on the tortured soul of mathematician John Nash, author of the "Nash Equilibrium." Somewhat oversimplistically put, Nash's game theorem holds that when every player in a multilayered contest possesses a known strategy and any change to each strategy is of no individual benefit, then an equilibrium is achieved.
Or, as the NY Times puts it:
While some of Mr. Obama’s advisers want him to impose sanctions against whole sectors of the Russian economy, the president has decided against it for now, cognizant of the resistance of European nations that have far more at stake economically, officials said.
And as the president put it:
The notion that for us to go forward with sectoral sanctions on our own without the Europeans would be the most effective deterrent to Mr. Putin, I think, is factually wrong.... For example, say we’re not going to allow certain arms sales to Russia--just to take an example--but every European defense contractor backfills what we do, then it’s not very effective.
As long as the Europeans see no good reason to change their strategy, Obama has no good reason to change his strategy. The problem? Nor does Putin.