"[The king] shall read from [his personal sefer Torah] all the days of is life, so that he shall learn to fear Hashem" (Devarim 17:19). There is no amount of time after which we can say that we have learned enough Torah. It requires a concentration of all our time – all our life – no less. Study of how to fear Hashem is a course that requires a lifetime, not just for people on a low level, but even for people on the highest level.
There are two types of fear, both of which appear in our parasha: one is desirable, the other is undesirable. The fear of Hashem that comes with study of Torah throughout life is obviously desirable. Later on we read about the man "who is fearful and has a soft heart," who has to "go and return to his house" (ibid. 20:8), instead of taking part in the nation’s battle. Just as one has to toil to create the desirable fear, so must one toil to rid himself of the undesirable fear. Furthermore, the two fears are opposed and contradictory – when one possesses the proper fear, it destroys the improper one.
Yeshaya asks: "Who are you that you fear a person who is destined to die?" (Yeshaya 51:12). Fear is present not only at times of war but even in times of peace. Most of our actions and our energies are actually motivated by mundane fears. We are afraid of what we will have to eat tomorrow and into the future. It seems to be the work of the Satan that a person wants to make shoes that will last him for seven years when he is not sure that he will live more than seven days, so that the Satan can then laugh at us. We are afraid of those who are far away from us and those who are close to us, lest they encroach on our space and take that which we were wanted for ourselves. Why do we work so hard for things that we do not need but are there just to glorify us, such as nice clothes and fancy drapes? All that hard work, which is motivated by a fear of what others will think, is a fear of people who are destined to die.
The Torah teaches us not to bow to pressure, but to realize what it means to be a free person. The Torah says about the king: "He shall learn to fear Hashem," and the Rabbis point out that this is referring to the king who has no one above him other than Hashem (Horiyot 11a). Let us remember that, in essence, all of Israel are the sons of kings (Bava Metzia 113b). If someone lacks fear of Hashem then he will have the fear of every leaf that falls off the tree. Only the significant, positive fear will straighten our spine; lowering our posture before Hashem can straighten our stature when it comes to other people.
The soldier who is fearful and soft of heart, say Chazal, is "one who is fearful because of the sins in his hand" (Mishna, Sota 8:5). This is the one who ‘shoots arrows up at Hashem,’ and kicks with a callous heart everything that is holy and pure, with the chutzpa of one who says that he has no Master. He does not know that when he denies his one Master, he is really creating for himself hundreds of thousands of human masters to fear.