Whenever this question is asked, the implication seems to be that war in the name of Christ is contrary to His mandate to “turn the other cheek” (Matthew 5:39).
Otherwise, why would anyone argue with the right to fight a religious war? To understand the “big picture,” we must first understand that the “turn the other cheek” passage is part of the Sermon on the Mount. Its messages pertain to individual believers. His famous sermon shows how believers should respond to temptation, injustice, and the everyday trials of life. It says nothing about warfare.
For some instruction regarding this, we must go to Romans 13, which discusses some of the purposes of government as well as the proper relationship between believers and government:
"… For it [government] is a minister of God to you for good. But if you do what is evil, be afraid; for it does not bear the sword for nothing; for it is a minister of God, an avenger who brings wrath upon the one who practices evil." (Romans 13:4)
God has given the state the power of life and death over its subjects in order to maintain order. Government also has the right to protect its people from evil by wielding the same “sword” in warfare. Some will argue that God has commanded, “Thou shall not kill.”
Therefore Christians cannot engage in warfare. Actually, God’s command was against murder, not killing. Nowhere does the Bible say that believers should not join the military and thus, should not participate in defending their country. Numbers 25:8-13 is one of many Biblical accounts where killing was required to eradicate sin. However, the bottom line is that while the God of the Bible is a righteous judge, He is also a God of love and mercy Who detests the shedding of innocent human blood.