Ephesians 6:10-13 said:
Finally, be strong in the Lord and in the strength of His might. Put on the full armor of God, so that you will be able to stand firm against the schemes of the devil. For our struggle is not against flesh and blood, but against the rulers, against the powers, against the world forces of this darkness, against the spiritual forces of wickedness in the heavenly places. Therefore, take up the full armor of God, so that you will be able to resist on the evil day, and having done everything, to stand firm.
For the Christian, warfare is an unfortunate product of the Fall in Genesis 3 and is not to be encouraged. While we will fight to defend our respective countries from other countries more infused with the sin nature, ultimately we want to fight the corruption inside of humanity and bring them into knowledge of the salvation available through Jesus Christ. If the unbeliever is alive, he or she has a chance to accept the Gospel; if the unbeliever is dead, they cannot be saved. Thus, warfare is not in our better interest as Christians.
I’ve quoted Ephesians 6, which is a portion of our Scripture that explains the matter. As Christians, we battle Satan and his lies and schemes, the world’s mechanisms to spread lies (the media, schools, and so on), evil philosophies, and tyrannical authorities that force people to believe lies or die. These are listed out in the verses. This is probably what the song you were quoting in the title of the thread refers to. “Onward Christian solidiers, marching
as to war.” “as” is the key word.
Another example: take an Islamic terrorist. We do not oppose the Islamic terrorist’s flesh and blood - that is, we do not want to kill him or her. We oppose the Islamic terrorist philosophy, Islamic fundamentalism, and we believe that terrorist is held captive to this evil philosophy. If they were to turn from their evil terrorist beliefs and accept the Gospel, we would welcome them to the family of believers, if their repentance and turning from evil was indeed true. However, if they persist in their evil, we believe that sin has consequences, the wages of sin is death, and we will defend ourselves. Hopefully that explains the situation for you.