You have to admit the viet cong were cunning,they dropped everything on them,including poison to clear the vegetation so that they could see the enemy,but they used to dig huge tunnel networks and hide (agent orange I think it was)
Its similar to north/south korea are today exept no war
The tunnels were a particularly clever weapon.
The Vietnamese were physically much smaller than the Americans and the tunnels were dug so that they were only just big enough for the vietnamese to be able to get through.
The US forces were not able to clear the tunnels because they simply could not negotiate them