Re: About two-thirds of Vietnam veterans were volunteers, not drafted.
Without a mandatory draft they would have had a very, very hard time finding ground combat people or Army pilots for that war. The folks that liked it were the career people ("It's not much of a war but it's the only war we have") and some of the Navy folks.
Up to that time one had to have a combat ticket punched to make General officer. All the Air Force pilots were volunteers. I don't know about the Navy and Marine Corps pilots.