What is the theme in The Killing Zone: My Life in the Vietnam War by Frederick Downs?

The difficulties of war is another theme. The American forces have the best technology available at the time. If soldiers are wounded they can be airlifted by a helicopter to immediate medical care. American air power can kill hundreds of enemy soldiers without risking American lives. Yet against a determined and entrenched enemy, this is not enough to win a quick victory. The enemy is able to disappear during the day and then at night mine the roads. American forces are forced to search for the enemy in the jungle because that is where the enemy hides out and conceals their battle reserves.