In 1970, the same fate befell another camp at Dak Seang. What had once been a supply corridor now stretched for from east to west. In early-February 1971, 16,000 (later 20,000) ARVN troops rolled across the Laotian border along Route 9 and headed for the PAVN logistical center at Tchepone. "Operation Lam Son 719", the long-sought assault on the Ho Chi Minh trail itself and the ultimate test of the U.S. policy of Vietnamization, had begun. Unfortunately for the South Vietnamese, U.S. ground troops were prohibited by law from participation in the incursion, and the U.S. was restricted to providing air support, artillery fire, and helicopter aviation units. At first the operation went well, with little resistance from the North Vietnamese. By early-March 1971 the situation changed. Hanoi made the decision to stand and fight. It began to muster forces which would eventually number 60,000 PAVN troops as well as several thousand allied Pathet Lao troops and Lao irregulars, outnumbering the ARVN by almost three to one. The fighting in southeastern Laos was unlike any yet seen in the Vietnam War, since the PAVN abandoned its old hit-and-run tactics and launched a conventional counterattack. The PAVN first launched massed infantry attacks supported by armor and heavy artillery to crush ARVN positions on the flanks of the main advance. Coordinated anti-aircraft fire made tactical air support and resupply difficult and costly, with 108 helicopters shot down and 618 others damaged. PAVN forces began to squeeze in on the main line of the ARVN advance. Although an airborne assault managed to seize Tchepone, it was a useless victory, as the South Vietnamese could only hold the town for a short period before being withdrawn due to attacks on the main column. The only way the invasion force managed to extricate itself from Laos was through the massive application of U.S. air support. By 25 March 1971 the last ARVN troops recrossed the border, closely followed by their enemy. As a test of Vietnamization, "Lam Son 719" failed; one-half of the invasion force was lost during the operation. South Vietnamese troops were poorly led and the elite Ranger and Airborne elements had been decimated. "Lam Son 719" did manage to postpone a planned PAVN offensive against the northern provinces of South Vietnam for one year.