US Army Lieutenant General H.R. McMaster looks on as US President Donald Trump announces him as his national security adviser at his Mar-a-Lago resort in Palm Beach, Florida, on February 20, 2017. / AFP / NICHOLAS KAMM (Photo credit should read NICHOLAS KAMM/AFP/Getty Images)
U.S. Secretary of Defense Jim Mattis (L) shakes hands with British Secretary of State for Defense Michael Fallon during a meeting at NATO headquarters in Brussels on February 15, 2017.
NATO allies meet new US Defence Secretary James Mattis for the first time in Brussels seeking reassurance over President Donald Trump's commitment but bracing for military spending demands. / AFP / POOL / Virginia Mayo (Photo credit should read VIRGINIA MAYO/AFP/Getty Images)
US President Donald Trump announces US Army Lieutenant General H.R. McMaster (L) as his national security adviser at his Mar-a-Lago resort in Palm Beach, Florida, on February 20, 2017. / AFP / NICHOLAS KAMM (Photo credit should read NICHOLAS KAMM/AFP/Getty Images)