The Tri Phap sat at the junction of infiltration corridors running from Cambodia deep into the Mekong Delta. Corridor 1-A crossed the Cambodian frontier near the border between Kien Phong and Kien Tuong Provinces, threading through the canal-laced Plain of Reeds before terminating in this swampy wasteland -- designated Base Area 470 by allied intelligence. A branch of corridor 1-B entered from the Parrot's Beak region of Svay Rieng Province to the northeast. Over decades, the terrain had been transformed into a network of permanent fortifications and hidden supply caches, all concealed beneath scrub vegetation in land too waterlogged for cultivation. After the January 1973 ceasefire, South Vietnamese forces in Dinh Tuong Province knew the PAVN was building up strength in the Tri Phap, but they lacked the troops to challenge it while simultaneously defending populated areas along Highway 4.

Captured documents and prisoner interrogations painted a clear picture through late 1973. The Z-18 Regiment had moved into the Tri Phap from Cai Bay District. Aerial photography revealed rice paddies planted north of the base -- the North Vietnamese were trying to become self-sustaining in the delta. ARVN units spent months grinding down four main-force regiments in the area, and by December 1973, nearly 40 percent of all PAVN soldiers killed in the second half of the year had died in delta fighting. But the real alarm came in January 1974: elements of the PAVN 5th Division were being ordered south from Tay Ninh to reinforce the Tri Phap. Two captured soldiers confirmed that a battalion of the 5th's 6th Regiment had already been dispatched. General Nghi understood the arithmetic -- if the 5th Division dug into the Tri Phap's prepared positions, no force available to IV Corps could pry them out.

On February 12, 1974, the ARVN 12th Infantry Regiment of the 7th Division, reinforced with armored cavalry, attacked from the east. Three days later, the 14th Infantry Regiment of the 9th Division drove in from the west, linking up on the Tri Phap's western edge. On February 19, the 10th Infantry struck from the south, completing the envelopment. Caught inside the closing ring, the North Vietnamese suffered devastating losses, particularly among rear-service and logistics troops. Over 500 PAVN soldiers were killed in the first week alone, and the ARVN captured tons of ammunition and nearly 200 weapons while sustaining comparatively light casualties. The fighting continued through February and March as the PAVN reinforced with the Dong Thap 1 Regiment, but the ARVN maintained relentless pressure, killing another 250 and capturing equivalent numbers of weapons in successive weeks.

Unable to reverse their battlefield losses, the PAVN resorted to terror. On March 9, they dropped an 82mm mortar round into the primary schoolyard at Cai Lay while children were lining up for class. Twenty-three children died instantly and 46 were wounded. In Bac Lieu, Viet Cong threw a grenade into a religious service, killing nine and wounding 16. On May 4, mortar shells struck a school in Song Phu, Vinh Long Province, killing six children and wounding 28. These attacks against civilians underscored the desperation of a force that could not match the ARVN in the field. By the end of March, over 1,100 PAVN had been killed in the campaign against fewer than 100 ARVN dead. Nearly 5,000 tons of rice were seized, along with more than 600 weapons and eight tons of ammunition.

The ARVN immediately began constructing fortified positions in the captured Tri Phap, installing four Regional Force battalions to hold the swamp permanently. The PAVN counterattacked on April 26 with two battalions, briefly overrunning an outpost, but the ARVN 14th Regiment and 2nd Armored Cavalry routed them with heavy casualties. By the first week of May, South Vietnamese forces controlled the Tri Phap firmly. The strategic payoff was significant: denied this base, the 5th Division could not extend operations southward into Dinh Tuong Province or westward toward Saigon through Long An. The PAVN was forced to build up on the periphery of the Parrot's Beak instead, where the ARVN countered with armored thrusts into Svay Rieng. It was a rare South Vietnamese offensive victory in the war's final years -- one that bought time but could not, ultimately, change the outcome that came the following spring.