There’s only one reason the US will deploy the THAAD missile defense system in South Korea, Defense Secretary James Mattis said Thursday as he arrived in the country on his first overseas trip as Pentagon chief. “THAAD is for defense of our allies’ people, of our troops who are committed to their defense and were it not for the provocative behavior of North Korea we would have no need for THAAD out here,” Mattis said, referring to the Terminal High Altitude Area Defense anti-missile system.