As I heard in a discussion with Navy Chief Petty Officers, you have to be smart enough to follow commands and know WHY you must follow commands during wartime. Otherwise you dilute your warfighter's effectiveness. That is why the military doesn't want people with low IQ.
Many years ago (over 50), I went downtown to the office where the military draft was managed. Back then, on Canal Street in New Orleans and about 4 long blocks from the river banks. We all had to take the written test and some physical tests. I had been deferred for service because I was in college and the military liked college grads as potential officer candidates. However, when my college deferment expired (because I got my Bachelor's degree). The written test was no biggie. I think I got one thing wrong because they had a picture of a tool I had never seen before and didn't know what you would do with it.
Among my test group, one young man sat still, looking forlornly at his test sheet where he had not even entered his name. I truly felt sad for the young fellow because when the time was up and the sergeant picked up the sheets, I overheard a bit of the conversation. The guy couldn't read. He had gone through school with a learning disability and never managed to learn to read! I had a chance to ask the sergeant what would happen to him. Basically, at the time they WERE taking such people into a special training track that, if he stayed with it, would teach him some necessary skills including how to read. However, the sergeant also offered the opinion that the recruit was SO hampered by learning disabilities that he was not going to be accepted for service. That thought was sobering. I wished him well and privately grieved for a person who was going to have a very hard life.
At that time the military draft in the USA was a lottery. They drew birthdays against numbers 1-366 chosen in random order so that being drafted and sent to Vietnam was literally the luck of the draw. They drafted numbers up to 183 that year, but my draft lottery number was over 250 so they left me alone. That is why I continued on into graduate school.