Matthew Henry on Deuteronomy 20:1-9

In the wars wherein Israel engaged according to the will of God, they might expect the Divine assistance. The Lord was to be their only confidence. In these respects they were types of the Christian's warfare. Those unwilling to fight, must be sent away. The unwillingness might arise from a man's outward condition. God would not be served by men forced against their will. Thy people shall be willing, Ps 110:3. In running the Christian race, and fighting the good fight of faith, we must lay aside all that would make us unwilling. If a man's unwillingness rose from weakness and fear, he had leave to return from the war. The reason here given is, lest his brethren's heart fail as well as his heart. We must take heed that we fear not with the fear of them that are afraid, Isa 8:12.