On 4 February 1971, Lieutenant general Đỗ Cao Trí led the first of 16,000 Army of the Republic of Vietnam (ARVN) troops into Cambodia. Initially the troops concentrated on clearing Highway 7 between Snuol and Chup, and for the most part the PAVN avoided contact.[1]: 47
On 26 February, however, advancing ARVN troops came under fire from a line of bunkers concealed in a tree line. Captain Jon E. Swanson of the 1st Squadron, 9th Cavalry Regiment, swept down in his OH-6A scout helicopter to engage the enemy with grenades and machine guns. After destroying five bunkers he came under fire from an antiaircraft machine gun. Despite having expended his grenades, he engaged the position with machine guns and knocked it out. When a second antiaircraft gun opened fire, hitting his helicopter, he attacked again before directing an Army helicopter gunship to destroy the target. He was then engaged by a third antiaircraft gun. Ignoring his own safety he flew his damaged and now virtually unarmed helicopter toward the enemy to mark the target for the gunships when enemy fire caused his helicopter to explode, killing him and his copilot. Swanson was posthumously awarded the Distinguished Service Cross for his actions that day. In 2002 the award was upgraded to the Medal of Honor.[1]: 49
Trí had been killed in a helicopter accident on 23 February and his successor, General Nguyễn Văn Minh, was less aggressive. This combined with the South Vietnamese focus on Operation Lam Son 719, which had begun on 8 February but had bogged down, meant that the momentum behind the incursion had started to wane.[1]: 49
The first large engagement occurred on 17 March, when two South Vietnamese task forces, supported by U.S. Army gunships, engaged a PAVN regiment in a two-day battle in the Chup Plantation, a major enemy depot. Two weeks later, a South Vietnamese task force defeated an enemy battalion in a five-hour battle near Suong.[1]: 49
After some additional activity around Snuol during the first week in April, the PAVN went to ground. Thereafter, ARVN troops focused on uprooting enemy supply caches. The operation remained relatively uneventful until the PAVN learned that the South Vietnamese planned to withdraw a task force of the 5th Infantry Division from Snuol in late May. The Battle of Snuol began on 25 May when three PAVN regiments massed to cut Highway 13, thereby trapping the task force in Snuol. An ARVN task force was sent up Highway 13 to relieve the besieged the task force, but the operation became a rout and the ARVN suffered heavy losses of men and equipment as they withdrew.[1]: 49 The battle rendered the ARVN 5th Division combat ineffective in the estimation of its American advisers.[2]: 338–9 According to II Field Force, Vietnam commander Michael S. Davison, the 5th Division troops were close to mutiny.[3]: 478
South Vietnam
Khmer Republic
United States
North Vietnam
Khmer Rouge