Well, people fall into two categories on this one.
The first interpretation is known as Just War, in that we are commanded to obey the authorities placed on earth by God (Romans 13:1), and many times in Old Testament history, God commanded his chosen people to wage wars against many peoples, in fact, even waging a war of conquest and genocide in securing the Promised Land.
The second interpretation is known as Pacifism, or the belief that all war is evil. The basis for this is usually 1 Cor. 13, in which we are shown that Love is the highest virtue, and adherents to a Pacifist interpretation view war as something that can never be loving.
I fall into the Just War mentality, and, to quote a scriptural-interpretation ministry’s view:
“Jesus marveled when a Roman Centurion (officer in charge of one hundred soldiers) approached Him. The Centurion’s response to Jesus indicated his clear understanding of authority, as well as his faith in Jesus (Matthew 8:5-13). Jesus did not denounce his career. Many Centurions mentioned in the New Testament are praised as Christians, God-fearers, and men of good character (Matthew 8:5,8,13; 27:54; Mark 15:39,44-45; Luke 7:2,6; 23:47; Acts 10:1,22; 21:32; 22:25-26; 23:17,23; 24:23; 27:1,6,11,31,43; 28:16).”