Many articles mention that Ukraine is a test for Obama. Usually they mean that the test is whether he is tough enough. In fact the test is the opposite: whether he can resist the pressure of the Republican extremists to escalate the situation.
There is always pressure on presidents who are generally perceived as weak - specially the liberals - to show that they are strong. Often they succumb. Kennedy had his Pigs Bay invasion. Johnson had his escalation in Vietnam. And by making trouble in Libya, Syria and Ukraine Obama is already near the top of the list of most aggressive presidents ever.
It is easy to have liberal ideals. But liberals often lack a clear vision of what the national interests are and that makes them an easy victim for extremists who claim the aren't doing enough to protect those interests.
In Ukraine the Russians have two interests: the position of the Russian population and the Russian bases. As neither is seriously disputed in the West there shouldn't be a problem.
Unfortunately US foreign policy is in the hands of neocons like Victoria Nuland. Like all bullies their aim is to harm Russia no matter how and they are boasting against each other about their achievements. Obama has given them a free hand and they have achieved their regime change and they are now pushing their new puppets to take some anti-Russian measures.
Now they have crossed Russia's red lines and Russia has taken some steps to make sure everyone understands that it means business. Obama reacts like a deer in headlights. He seems incapable of taking the initiative back and formulate some sensible policies and instead allows himself to be caught in a senseless discussion about sanctions.