After Entering Crimea, Where Will Russia Stop?

With Russian forces now effectively in control on the Crimean peninsula, there's an ominous question on the minds of people around the world: Will President Vladimir Putin go further — sending his troops into other parts of Ukraine? If he does, tensions that wouldn't seem able to go much higher invariably would ratchet up, and Russian actions that so far haven't led to any shots being fired might lead to war between Russian and Ukrainian forces.
 British Foreign Secretary William Hague on Monday said that what's happening in Ukraine is already the "biggest crisis in Europe in the 21st century."