What should obama do?
It's too late for him to do anything. The time to act was long ago, like at the time of the reset button debacle.
What not to do is offer military aid to the failing Ukranian coup. Such an action will turn Ukraine into Syria. This is ultimately a civil war. We can help in the murder of another 200,000 people or let the Ukrainians vote their way out.
And as I predicted, another "what can we do" handwringing. There is plenty besides sit back and watch Putin put the Soviet Union together.