When the Jewish people went to battle, they were given an inspirational talk by a special Kohein. He was known as the כהן משוח מלחמה, the Kohein anointed for battle.
He reminded the army that Hashem is in their midst, and He is fighting their battle with them. There were exemptions given to soldiers who have not yet completed their homes, betrothed a wife, or has not dedicated his vineyard.
The officers then announce that the soft hearted and fearful individuals, must not take part in battle. There are several explanations in defining this condition of simply being afraid.
The Torah itself gives the reason that such a person will harm the overall morale of the army. There is another explanation that the fear was the concern that he did not have enough merits, that would protect him in battle.
It is quoted in the name of Rabbi Akiva that this soldier needed to go home because he was faint hearted. He could not handle the sounds of the horses, and seeing swords drawn. It was simply too much for him.
A final explanation is from the Tosefta of Masechet Sotah. It refers to one who is a brave warrior and very strong, but he is overly merciful. His misplaced pity on the enemy, makes him a liability to the Jewish army. He must be sent home.
The key factor in succeeding in battle, was the trust in G-d that was needed. The ideal was to choose righteous individuals to fight on behalf of Am Yisrael. This would guarantee the greatest results.