Operation Junction City was one of the largest American operations of the Vietnam War, a massive search-and-destroy sweep through Tay Ninh Province aimed at dismantling Viet Cong infrastructure near the Cambodian border. On March 26, 1967, Haig's 1st Battalion, 26th Infantry was ordered deeper into War Zone C for an airborne assault. The plan called for landing at a clearing designated LZ George, some 14 kilometers west of their fire support base at Sroc Con Trang, then securing the zone for a follow-up landing by the 1st Battalion, 2nd Infantry Regiment. Poor weather delayed the operation by a day. When Haig's men finally touched down on the afternoon of March 30, they found open fields of tall, meadow-like grass surrounded by medium to heavy jungle. Cloverleaf patrols uncovered fortified positions around the landing zone but made no contact. That night, the battalion dug fighting positions with full overhead cover and interlocking fields of fire. The quiet was deceptive.

The next morning, March 31, the second battalion landed without incident and moved two kilometers southwest. Haig's companies fanned out on search operations. Then, at 13:00, the reconnaissance platoon walked into the ambush that killed Hill. When word reached the B Company commander that the platoon was desperate and Hill was down, he turned his men north to help without informing Haig. The gesture was brave but uncoordinated. Haig learned of B Company's move only after boarding his helicopter, and by then both the reconnaissance platoon and B Company were heavily engaged against a force of at least battalion strength. The B Company commander reported rockets, mortars, recoilless rifles, and heavy machine gun fire. His men were pinned down and running low on ammunition. Haig landed near the fighting, found Hill's body and the B Company commander wounded. Company A pushed through and gained fire superiority, allowing the embattled units to withdraw under artillery and air cover. Seven Americans died and 38 were wounded in the afternoon's fighting.

At 04:55 on April 1, a lone mortar round exploded in front of the battalion perimeter. Most commanders might have dismissed it as a stray shot. Haig recognized it instantly as a registration round, a ranging shot fired to calibrate a barrage. He ordered every company to full alert and into cover, then recommended the neighboring 1st Battalion, 16th Infantry do the same. Five minutes later, hundreds of 60mm, 82mm, and 120mm mortar rounds rained onto the northern perimeter. The incoming was so dense, the Americans said the mortars sounded like loud, heavy machine guns. Because of Haig's early warning, only 12 men were wounded in a bombardment that could have been devastating. At the same time, a coordinated mortar and howitzer attack struck Fire Support Patrol Base C, where much of the artillery supporting both battalions was positioned. The Americans were surprised the Viet Cong failed to attack Fire Support Patrol Base Thrust, which continued providing unhindered fire support throughout the battle.

Seven minutes after the mortar barrage lifted, the ground attack hit the northeast edge of the perimeter. Three battalions of the VC 271st Regiment and elements of the 70th Guard Regiment threw themselves at Companies B and C of the 1/26th Infantry. Withdrawing listening post soldiers accidentally triggered flares, silhouetting themselves and drawing withering fire. The Viet Cong overran three bunkers and seized a 40-by-100-meter section of the Company C perimeter. Hand-to-hand fighting erupted in the darkness. Captain Brian H. Cundiff of Company C defied the fire to move among his men and organize a defense. At 06:30, the reconnaissance platoon moved into a blocking position, and alongside B Company, began fighting to restore the original line. Air strikes came over the target area four times per hour. The decisive weapon proved to be cluster bombs, which could be released at low altitude within 30 meters of friendly positions. As the ordnance found its mark, large groups of Viet Cong fell. Many began running, throwing down their weapons. By 08:00, Cundiff led a counterattack that pushed the remaining VC back into the waiting artillery barrages, and the perimeter was restored.

The numbers from Ap Gu are staggering. Alpha Battery, 1st Battalion, 7th Artillery fired approximately 15,000 rounds during the battle. Air Force fighter-bombers flew 103 sorties, dropping over 100 tons of ordnance. The Americans reported 17 killed and 102 wounded; they claimed 609 Viet Cong dead, with 491 bodies found in the base area alone. Haig credited air power, particularly the cluster bombs, as the decisive factor, while acknowledging that without the combined effect of artillery, mortars, helicopter gunships, and infantry, the defense would have failed. He later claimed that Ap Gu demonstrated the Viet Cong were tactically naive in large-unit open combat. Perhaps. But it also demonstrated something else: that an alert commander who correctly reads a single mortar round, who orders his men into cover sixty seconds before a barrage that would have caught them exposed, can turn the balance of a battle before it even begins.