Gen Martin Dempsey denounces the president's threat to use troops to suppress ongoing civil unrest.
Read moreJames Mattis
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Video caption: James Mattis mocks Donald Trump at gala dinner The former US defence secretary hits back after the president describes him as "the world's most overrated general".
By Jonathan Marcus
Diplomatic correspondent
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Video caption: 'Syria withdrawal one bridge too far for Mattis' Former US Defence Secretary William Cohen reacts to the resignation of Jim Mattis.
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Video caption: Mattis to Bolton: I hear you're the devil incarnate US Defence Secretary James Mattis jokingly tells new National Security Adviser John Bolton he's heard he is "the devil incarnate".