In mid-March U.S. intelligence learnt that the People's Army of Vietnam (PAVN) 27th Regiment was moving south of the DMZ in an attempt to cut Route 9. A counter-infiltration operation was planned whereby the 3rd Squadron, 5th Armored Cavalry and the 1st Battalion, 11th Infantry Regiment would patrol the Khe Chua Valley north of Firebase Fuller.
The operation commenced on 23 March with the 3/5 Cavalry moving into the Khe Chua Valley meeting limited opposition. On the morning of 24 March the Cavalry encountered an estimated Battalion-size PAVN force in entrenched positions and pulled back to allow for artillery support, the Cavalry called for reinforcements and Company I 3rd Battalion 9th Marines was lifted by helicopter to the scene of the engagement. The following day the Cavalry and Marines continued patrolling westward into the valley while Companies A and B of the 1/11 Infantry were deployed by helicopter in the north of the valley. At 03:00 the PAVN attacked the 1/11 Infantry night defensive positions causing limited damage, at dawn the Infantry assaulted in the direction of the PAVN attack returning to their previous defensive positions at nightfall.:59
On 27 March the 1/11 Infantry resumed their attack supported by 3/9 Marines, the Infantry engaged PAVN in bunkers and fell back to allow for artillery support and Company C 1/11 Infantry was deployed by helicopter to form a blocking position. The Infantry resumed their attack but were hit by PAVN mortar fire and counter-attacks and so withdrew to allow for further artillery and airstrikes before taking the PAVN positions. Total losses for the day were 120 PAVN killed for the loss of 13 U.S. killed including all of Company B's officers.:59
On 29 March Company D 1/11 Infantry overran a PAVN bunker complex killing 5 PAVN. As Company A advanced west of Company D it came under attach from an estimated Company-size PAVN unit, the day-long battle that followed resulted in 30 PAVN killed.:60
The Cavalry, Infantry and Marines continued patrolling the Khe Chua Valley for a further 4 days meeting little resistance.:60
Operation Montana Mauler concluded on 3 April, the PAVN 27th Regiment had lost approximately 300 killed.:60
This article incorporates from websites or documents of the United States Marine Corps.