Pool/Matt Dunham via Reuters WASHINGTON (Reuters) - U.S. Defense Secretary Jim Mattis said on Friday that any military solution to the North Korea crisis would be "tragic on an unbelievable scale" and Washington was working internationally to find a diplomatic solution. North Korea has defied all calls to rein in its nuclear and missile programs, even from China, its lone major ally, calling them legitimate self-defense. It has been working to develop a nuclear-tipped missile capable of striking the U.S.