The US and Iran: Diplomatic relations turn icy again

Responding to his first challenge from the world's most volatile region, President Trump has invoked economic sanctions against Iran. That's apparently what he meant when he said Iran was "on notice" after it tested a ballistic missile last weekend.

It's not clear if he plans more to come. But there are stark warnings of possible escalation, unintended or otherwise, as Trump continues to keep Washington guessing, along with the rest of the world. During his campaign he promised to be "unpredictable," and today he tweeted that, "Iran is playing with fire." Iran says he has a lot to learn.