In January, just one month before North Korea conducted its third nuclear-weapons test, 187 U.S. Marines launched a five-week military-training exercise with South Korea by racing bare-chested down a snow-covered hill in below-freezing temperatures. This joint mountain-warfare training has occurred in South Korea every year since the end of the Korean War to prepare troops to defend the hostile region year-round and to build camaraderie. "The differences between our two [groups] is small," says Capt. Chris M. Frey from Texas. "It has been an awesome experience getting to work with the Republic of Korea Marine Corps." The visiting U.S. Marines – stationed in Hawaii – camp in subzero temperatures, rappel down snow and ice, snowshoe while simulating battle formations, and learn how to carry gear on cross-country and downhill skis. "The unit is only as strong as your weakest skier," says Frey. Since only half of the American soldiers have previously skied, they have to work to get in shape for the slopes, waking up at 6 am every day for a regimen of lunges, squats, and crunches.