By 1943 the War Dept was concerned enough to commission a WWI infantry officer who was by then a BG to investigate why 1/3 of the recruits/draftees ended up getting "unfit for service discharges" from the military medical/psychiatric DRs before they could further serve in the military. The book is named: "All but Me and Thee" by BG Elliot D. Cooke. Published in 1946, it's a short, good read.
Essentially, the conclusion was that 1/3 of eligible (physically fit) males were unfit morally; 1/3 unfit mentally; and of the remaining 1/3, most made good soldiers until exhausted, chronically or circumstantially poorly lead or suffered what is now known as PTSD from individual or repeated "bad luck".
I don't think those numbers have really changed much but any lack of enthusiasm from the populace isn't going to help.