Adm. James Winnefeld Jr., commander of North American Aerospace Defense Command and U.S. Northern Command, visits the Muscatatuck Urban Training Center in Butlerville, Ind., July 16 to observe the Vibrant Response 10.2 field training exercise, a national emergency response exercise aimed at preparing first responders for the unthinkable – a nuclear blast in a major U.S. city. The exercise is facilitated by U.S. Army North's Joint Task Force – 51, headquartered in San Antonio.