Complex sentence: When night falls, bringing a hellish downpour, the Viet Cong and elements of the North Vietnamese Army attack the fortification, this opportunity to attack, as the storm has kept Air Support entirely out of the realm of possibility.
Split into simpler sentences: When night falls, bringing a hellish downpour, the Viet Cong and elements of the North Vietnamese Army attack the fortification. They have realized this was the right opportunity to attack as the storm has kept Air Support entirely out of the realm of possibility.