Final answer:
During the Vietnam War, the US military primarily used helicopters to quickly move troops across the jungle for missions against the Vietcong and North Vietnamese forces.
Step-by-step explanation:
The US military moved troops quickly across the jungle during the Vietnam War predominantly in helicopters. This transportation method was essential for what was known as "search and destroy" missions, where the objective was to seek out and destroy the Vietcong and North Vietnamese troops and their supply lines. The Vietcong utilized guerrilla warfare, disguising themselves as civilians and using a network of tunnels for bases, making it difficult for US forces to engage them directly. Often, the US troops had to contend with the H-o Chi Minh Trail, a complex network of paths and tunnels used by the Vietcong and North Vietnamese to move soldiers and supplies, which was difficult to observe and target from the air due to its extensive cover.