evidence: On some of the world's harshest terrain, through the scorching heat of summer and the numbing cold of winter, American troops fought with steely determination and uncommon courage. As they gained ground, pushing the North Koreans back toward the 38th parallel, American families began to hope that our troops would be home by Christmas. But in November, at the Yalu River in North Korean territory, American forces encountered a new and daunting antagonist: Chinese forces had joined their North Korean allies, and the tide of battle turned once again.