The Dept of Defense was only created in the late 1940s. Before that the US had the Dept of War, the Dept of the Navy, and other organizations. The point of calling it "defense" was not because "everyone has the right to defense", but because the US was promoting the United Nations and waging a Cold War, and wanted to pretend that it would never do anything proactive or aggressive. That is, it was propaganda, as the current preferred name "Dept of War" is now for a different posture with regard to America's adversaries.
If you're going to call people stupid or immature for making certain decisions, maybe take a couple minutes to find out who made the decisions, and/or what the history of those and similar changes has been.
[1] https://www.ccac.gov/system/files/media/calendar/images/Semi...
[2] https://www.usmint.gov/coins/coin-programs/semiquincentennia...