Out of Fort Lewis on the 25th of September 1966 the Division was sent to the II Corps Military Tactical Zone (with the 3rd Brigade, 4th Inf sent to III Corps and the 25th Infantry Division).
Within days of it's arrival in Vietnam the 4th Infantry Division was involved in Operation Attleboro, an effort to located and destory the Viet Cong headquarters known to be in Tay Ninh Province on the Cambodian Border. Involved in Operation Junction City, Border patrol around Pleiku and Kontum. The Posion Ivy Division was involved in the 1970 Invasion of Cambodia before returning to Fort Carson in December 1970.
On return to Fort Carson, the 4th Infantry Division was, for the third time, De-activated.