The Battle of Tri Phap took place from 12 February to 4 May 1974 when South Vietnamese forces successfully launched a pre-emptive attack on a North Vietnamese base area in Dinh Tuong Province before North Vietnamese forces were scheduled to be moved to the area.
Background
One of three principal People's Army of Vietnam (PAVN) infiltration routes, corridor 1-A crossed the Cambodian frontier near the border between Kien Phong and Kiến Tường Provinces, traversed the maze of canals through the Plain of Reeds and ended in the watery wasteland called the Tri Phap (known as Base Area 470 by Allied intelligence) where those provinces join Dinh Tuong Province. A branch of corridor 1-B from the Parrot's Beak of Svay Rieng Province entered the Tri Phap from the northeast. A Viet Minh base established during the First Indochina War, the Tri Phap was partly covered with brush, with little land suitable for cultivation, essentially a swamp that over the years had been laced with permanent fortifications and hidden storage areas. No Allied force had succeeded in occupying or inflicting any serious damage to the installations or forces in the Tri Phap. Immediately after the 28 January 1973 ceasefire, South Vietnamese units in Dinh Tuong were preoccupied with maintaining security in the central and northern reaches of the province and could not divert the forces necessary to clean out the Tri Phap, even though they were aware of increased PAVN activity there.
A document captured on 9 August 1973 disclosed that the Z-18 Regiment of PAVN Military Region 2 was moving into the Tri Phap from Cai Bay District in northern Dinh Tuong Province and that it would probably be replaced in Cai Bay by the Dong Thap 1 Regiment. Information in the document pertaining to planned attacks in northern Dinh Tuong was confirmed by attacks on several outposts on 8 August. Furthermore, aerial photography showed that fields north of the Tri Phap had been planted in rice, part of the PAVN's effort to become self-sustaining in the Mekong Delta. With pressure mounting along Highway 4, however, IV Corps could not then challenge the PAVN activities in and north of the Tri Phap. Nevertheless, the ARVN repulsed, with heavy losses to the PAVN, numerous battalion-sized attacks against outposts and firebases in Cay Bay, Cái Bè and Sam Giang Districts during July and August. In the first week of September alone, PAVN casualties in the region were 144 killed, while those of the ARVN were 17 killed.
The surge in PAVN attacks which continued through November, was motivated in part, by the rice harvest and marked by PAVN attempts to gather as much of it as possible. But beyond that, the PAVN objectives were to protect the installations in the Tri Phap, expand the base area there, and use the infiltration corridors from Cambodia without interference from the ARVN. Success in these ventures would force contractions of the ARVN defenses along Highway 4, demoralize the soldiers of the ARVN 7th Division charged with the responsibility and support the propaganda campaign among ARVN troops.
As the year wore on, ARVN units slowly wore down the four main force regiments in PAVN Military Region 2 - the Z-18th, Z-15th, E-24th and DT1. Despite receiving hundreds of fresh replacements from the north, these regiments gradually lost ground to aggressive attacks. The PAVN 207th Regiment, which had suffered so badly in its disastrous Hồng Ngự campaign, was required to provide soldiers to replace losses in the E-24th Regiment. These demoralized soldiers were intercepted en route to the Tri Phap area in September; their casualties were heavy and 14 were captured. The PAVN 6th Division was disbanded in late 1973 and its depleted regiments were assigned to PAVN Military Region 2. The South Vietnamese Joint Operations Center provided data on casualties in December 1973 that showed nearly 40 percent of all PAVN killed during the last half of 1973 died in the delta. Although the figures were estimations the ratio was probably very close to reality, supported as it was by weapons captured and corresponding ARVN casualties.
The South Vietnamese held the Kien Tuong Province capital of Mộc Hóa and a base at Long Khot (), both of which were well within 105 mm howitzer range of PAVN artillery in Svay Rieng, but there were great reaches of uncontrolled, unoccupied territory between the Cambodian border and the first major population concentration along Highway 4 through Mỹ Tho. The PAVN 5th Division operated out of Svay Rieng Province in both directions, through Tây Ninh Province to An Lộc and south toward Mỹ Tho. In early 1974, the 5th was north of Tây Ninh but available for operations into Kien Tuong and Hậu Nghĩa Provinces.