Watch live: Aspen Security Forum discussion with Lester Holt and Gen. Mark Milley

Watch live: Aspen Security Forum discussion with Lester Holt and Gen. Mark Milley

NBC News’ Lester Holt hosts a live discussion with chairman of the Joint Chiefs of Staff, Gen. Mark Milley, on the military challenges of the 21st century. NBC News is the media partner of the Aspen Security Forum.