I had never heard of this until last night; in short, during the Vietnam era, and to a lesser extent in the Iraq/Afghanistan era, the military lowered their standards to a point where people with significantly subnormal IQs were admitted into the military, and usually given KP or other noncombat duties, if they made it through boot camp at all.
Here’s a 36-minute speech from someone who wrote a book about it.
If you watch it, you’ll also see a lot of pop-ups from the controversial psychologist Jordan Peterson where he discusses this and similar issues. Back then, intelligence was thought to be a more fluid thing than it actually is, and in the presentation, it’s mentioned that military officers thought they could raise IQ as much as 20 points with proper training, etc.
I do remember that in the “Unbroken” story, Louis Zamperini said that towards the end of the war, the Japanese soldiers he encountered became, as he put it, progressively stupider, so it wasn’t just the U.S. that has done this.