I was in the US Air Force (I didn't fly planes), and I remember my training instructors saying that the U.S. military presence posted along the South Korean border is symbolic and has no true physical application. If North Korea’s military invaded South Korea, the U.S. troops would easily get crushed due to their small numbers compared to the massive force of the N. Koreans. All of the U.S. troops would easily be killed. The broader U.S. forces (outside of S. Korea) would not have enough time to respond to the invasion of S. Korea by N. Korea. What would happen in the aftermath of the invasion is something entirely different, but the current “protection” of S. Korea by the U.S. military acting as “defenders” is a joke.