JMA:
This asks big questions of the USMC of that era as to where were all the NCOs who are the backbone of the Corps?
Carl:
Very young officers and NCOs was perhaps the best that could be done in Vietnam. As the war progressed the NCOs got younger and younger because IIRC all the older guys who started out weren't available anymore, many because they didn't want to face the prospect of deployment after deployment. Maybe the same thing with the officers. We had a lot of people over there for years. That added to the inequities of the draft system and there just wasn't that much to choose from.
JMA:
Talking about the older NCOs I noted that where they had been in the service before the war (meaning they joined up in peacetime for peacetime) tended to fall out early (meaning leave the service or find less onerous posts from where to see out the war). The younger ones who joined up during the war (or for the war) seemed to last a lot better. Similar back then for the US maybe?
Those who served in WW II -- most of the senior NCOs in the Marines and the Army -- hit 20 years service and retirement eligibility in 1962. Some retired but most continued to serve, headed for 30 years. When the US committed to Viet Nam in large numbers, the relatively smaller pre-Viet Nam services had NCOs scattered around the world in jobs that ranged from non-essential to essential. The units that deployed to Viet Nam in 1965-66 took a slew of experienced senior NCOs and most did fairly well. Those NCOs finished their tours in 1966-67 and returned to CONUS. The 'second string' of NCOs culled from around the World went to VN in 1966-67 and they did less well but at least they were there. By 1968-69, it was time for the 'third string' -- except there was none. The stopgap was to send graduates of the Non-Commissioned Officers Candidate course as SGTs to VN plus some returning senior NCOs from the 1965-66 era and a second tour. Good kids, do anything you asked but they didn't know much...

In the meantime, the world wide commitments did not go away; they were culled and cut but there were still plenty of requirements. In 1969 I volunteered to go to VN for a third tour but they called me and said as I had two tours, I would instead go fill a shortfall at either SHAPE or in the MAAG in Iran, my choice -- but VN was out for me. What had also happened was the folks who had completed a second tour decided to depart as had some after a single tour. Most of the WW II folks had over 25 years by 1967 and decided that WWI, Korea and A VN tour were enough. Those of us who'd been in Korea but not WW II couldn't have retired if we wanted to but we were sent to fiull those other requirements so that folks who had been in those slots could get a tour in the SEA War Games.

It all boils down to numbers -- demand exceeded supply due to casualties and retirements and the Army and Marines were directed to not hold anyone over involuntarily to avoid upsetting Voters. Lyndon didn't want to do that. In 2004, Bush did it regardless...