U.S. President Donald Trump's shake-up of his national security team adds to the burden on one man at the center of any decision about war and peace: Defense Secretary Jim Mattis.

Long a champion of alliances and diplomacy, Mattis increasingly finds himself surrounded by policy hawks on issues from Iran to North Korea. Yet his command of the nation's 1.2 million active duty personnel makes him uniquely placed to steer Trump away from any rash decision to unleash the U.S. military.

Trump stunned his own aides this month by reshaping his foreign policy team in a more hawkish bent ahead of a key decision on the Iran nuclear deal and a historic summit with North Korea's leader. With a tweet, he said he'd replace Secretary of State Rex Tillerson with CIA Director Mike Pompeo. Then Thursday he tapped former United Nations Ambassador John Bolton to be his third national security adviser in 14 months, dismissing H.R. McMaster.