It seems that what you want is something that retards human problem-solving skills. This can be done with disease or toxins.
Exposure to certain types of disease or toxins. Diseases like whooping cough, measles, or meningitis can cause intellectual disability if medical care is delayed or inadequate. Exposure to poisons like lead or mercury may also affect mental ability. (Source)
Difficulties with the idea
- There is no such thing as something that can retard problem-solving skills for one subject but not another. What gives us the ability creatively use a tool (e.g., how to drive a car somewhere we haven't been before), gives us the ability to understand the tool.
Said another way: if you take away our ability to understand the technology behind a car (gears, friction, leverage, combustion, electricity, etc.) you also take away high-volume farming, large density water distribution, disease control, etc. The underlying technology used to maintain society at the population densities enjoyed during WWII would very quickly turn cities into death traps (starvation, sanitation, disease).
Said in yet another way: Losing the ability to design, repair, and maintain a car means losing the ability to understand why you'd use the car in the first place. You wouldn't solve the problem of getting from A to B by thinking, "I can use a car to do that!" You'd simply walk there, having never considered the car as a possible solution to the problem.
It takes time. Nothing can retard brain functions in a moment, or even in weeks. It may take months of lead exposure to begin affecting a child, but it would take years to begin affecting a trained adult. Why? Because cognition is very complex, involving memory, sensory processing, abstraction and association, etc. A great deal of adult behavior has transitioned from learning to do something to the habit of doing something. This is why people with forms of dementia can drive cars (they may have no idea where they ended up, but they successfully operated the vehicle). This is because the skill of driving has become so habitual that it doesn't require nearly as much thinking as it does when learning how to drive.
Radiation isn't selective. Radiation strong enough to destroy the brain's higher functions is destroying the rest of the brain with them, including its ability to manage autonomic functions like breathing and blood circulation.
I believe forced mental retardation of a large population is plausible from the perspective of an engineered disease so long as you remember that such retardation will manifest in specific ways with very general results. Here's a few off the top of my head.
- Poor memory recall.
- Poor memory association.
- Poor memory retention (very similar to recall, but not quite)
- Poor abstraction (everything becomes very, very literal, necessary for mathematics and language)
- Poor sensory processing