AC 188: Colony L3 X18999
Brigadier General Septum permits young Specials officer Treize Khushrenada to lead three of his cadets (including 12-yr-old Lucrezia Noin) against rebels attacking the colony's Alliance Base. Treize is injured when he uses his mobile suit to block a bazooka shot aimed at the command center. While recovering in a local hospital, Treize has a brief affair with nurse Leia Barton - resulting in the conception of Marimeia Barton Khushrenada. (There is no official evidence - that I know of - indicating Treize ever knew of her existence.)
AC 191, April 8:
Specials Commander, General Catalonia, dispatches Treize (19) and Zechs (15) to crush the rebel terrorists who have stolen several prototype Aries suits from the Alliance's JAP point base. Treize is quite impressed with Zechs's piloting skill.
AC 193: At age 21, Treize is promoted to Commander of the Specials (and leader of the secret society OZ).
AC 194: General Septum orders the use of biological weapons to dispose of the fierce Long Clan on the aging L5 colony A0206. Unable to abide such cowardly and unchivalrous tactics, Treize orders the Specials to attack the colony instead. The attack is repulsed (largely by Chang Wufei in the still incomplete Shenlong Gundam) and the biological weapons are destroyed. (Long Mieran, granddaughter of matriarch Ron Shirin and wife of Chang Wufei, is killed in the attack.)