The President of the United States is serious this time. He was only joking about "crossing the line" in Syria.
The warning to Russia if from the bottom of his heart. instructing his Secretary of State to alert the Russian Army that any military intervention in the Ukraine would be "a grave mistake."
Yes, by golly, he means it this time.
Just wondering how he would punish the Russian Army for taking the entire Ukraine. They already took the Crimea, part of the Ukraine, in case you missed it.