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While President Donald Trump rattles sabers on Twitter and slams "Rocket Man" Kim Jong Un, there is also a perceptible hardening of tone among senior officials.
**This image is for use with this specific article only.**

While President Donald Trump rattles sabers on Twitter and slams "Rocket Man" Kim Jong Un, there is also a perceptible hardening of tone among senior officials.

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    Trump to Kim Jong Un: My nuke button is bigger than yours

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Trump to Kim Jong Un: My nuke button is bigger than yours

"He literally has in his hand -- when he gets on that smartphone -- history in his hands," says Admiral John Kirby. "And I don't think he...fully appreciates that responsibility."

Trump to Kim Jong Un: My nuke button is bigger than yours

"He literally has in his hand -- when he gets on that smartphone -- history in his hands," says Admiral John Kirby. "And I don't think he...fully appreciates that responsibility."