FM 72-20 Jungle Warfare(1944) by United States. War Department. General Staff

In jungle warfare, the soldier often fights two enemies: man and nature. The elimination of nature as an enemy and the use of the jungle itself as an ally are training objectives fully as important as the elimination of the human enemy. The soldier must be trained not to fight the jungle: he must be capable of living successf ully in it and making it work for him against the human enemy. The jungle is a strict taskmaster: unless an individual adjusts himself to the conditions imposed by jungle environment, he will be unable to exist long, even if there is no human enemy. It must be remembered that the difficulties imposed by jungle environment are impartially imposed upon enemy as well as friendly troops, and that the unit which better overcomes the difficulties imposed by the environment has a distinct advantage over the opposing force.