AdvertisementPresident Trump Now Owns These 7 National Security ChallengesJanuary 23, 2017Philip EwingfacebookEmailPresident Trump announces retired Marine Corps Gen. James Mattis as his choice for defense secretary, on Dec. 6 in Fayetteville, N.C. Trump, who was highly critical of Barack Obama's military and foreign policies during the presidential campaign, inherits a number of challenges. (AP)