Final answer:
North Vietnam supplied allied troops in South Vietnam via the HoChiMinh Trail, which was a network of hidden paths and tunnels resistant to U.S. air detection and attacks.
Step-by-step explanation:
North Vietnam was able to supply allied troops to South Vietnam through the use of the HoChiMinh Trail, a complex network of paths and tunnels that connected North and South Vietnam. The trail extended into neighboring countries Laos and Cambodia, making it difficult to be detected and attacked from the air.
Despite the use of defoliants such as Agent Orange and napalm by US forces, the HoChiMinh Trail remained an effective supply line for the Vietcong and the North Vietnamese Army (NVA), who used it to transport weapons, soldiers, and supplies, which sustained their guerrilla warfare efforts and contributed to their persistence in the Vietnam War.