Colonel David H. Hackworth, author of About Face, Vietnam Primer, and Steel My Soldiers' Hearts, wrote:
"Hayes was a great officer...and he had four attributes that made him a perfect CO for this particular war, not to mention for me. First, he had a great appreciation for the cutting edge, having fought in Korea for three years as a recon platoon leader and a rifle-company commander and been wounded four times. Second, he'd already spent four years or so in Vietnam, so he had a basic understanding of the Viets and the war itself. Third, having been commissioned as an Armor officer, his tactical orientation from the earliest days of his career was for shock action and flexibility, which is what I was into and what guerrilla warfare is all about. And finally, in addition to having formed and trained the first Ranger battalion in the Vietnamese Army in 1962, Hayes had had a lot of G experience [guerrilla] of his own in Southeast Asia, with the Special Forces Apache and Project Delta teams. ... John Hayes was a quiet, careful, methodical, and introspective soldier, just like the guerrilla enemy we were tasked to fight, and had, as an added bonus, a keen understanding of the Oriental mind.... Though promoted below the zone to colonel in 1968, when he turned down attendance to War College two years in a row, he was told by the chief of the Colonels' Division that he'd reached the end of the line careerwise. Apparently general officers, this guy said, didn't take kindly to people who considered continual involvement in a combat position in Vietnam more important than going to War College." (About Face, Simon & Schuster 1990, 681-82)
When Hackworth was Commander of US 4th Battalion/39th Infantry, he had Colonel John Hayes as his 1st Brigade/9th Division Commander in March 1969. Colonel Hayes went on to become General Hieu's ARVN 5th Infantry Division Senior Advisor.