At least in 1950 they weren't.
For Veterans Day today, my boys and I looked a little information about the Korean War (they were interested in that because some of their great-uncles served in Korea). We followed the action of the invasion by North Korea, the Allies coming in to help, and their successful recapture of South Korea. When we reached the point of the US/UN decision to cross the 38th Parallel, JG declared that that was a terrible idea.
Only after countless more lives were lost did President Truman conclude that they never should have crossed the line. Too bad they hadn't applied some fifth-grader logic before they did it.