While the Bible doesn't approve of war for every cause, and while it encourages peace with all persons (Rm 12:18), it nonetheless indicates that peace and justice sometimes require war (Mt 24:6). This is made clear from many considerations. First, the Bible does not prohibit all taking of life. For instance, killing in self-defense is justified (Ex 22:2), as is killing in capital punishment (Gn 9:6). Government is divinely authorized to use "the sword" (Rm 13:4), as Jesus Himself recognized (Jn 19:11). Second, under the law, God spelled out the rules of warfare for Israel (Dt 20). Third, while Jesus forbade His disciples from using a sword for spiritual purposes (Mt 26:52), He urged His disciples to buy a sword if necessary for protection (Lk 22:36-38). Fourth, John the Baptist did not say that armies should be abolished and did not call for repentance from serving in the office of soldier (Lk 3:14).
The Bible commands Christians to obey their government (Rm 13:1-7; Ti 3:1; 1 Pt 2:13-14). However; there are limitations to such obedience. When the government commands worship of idols or a king (Dn 3:6), forbids preaching the gospel (Ac 4-5), or orders the killing of children (Ex 1), then it is a believer's duty to disobey. Likewise, if government engages in unjust warfare, believers may dissent. However, like Daniel (Dn 6), the three Hebrew young men (Dn 3), and Peter (Ac 4-5), those who disobey government must accept the consequences meted out by the state.
Several conditions for just war are given in the Bible. First, it must be declared by one's government (Rm 13:4). Second, it must be in defense of the innocent and/or against an evil aggressor (e.g., Gn. 14). Third, it must be fought by just means (Dt 20:19).
In addition to the above reasons for a just war policy, biblical arguments for total pacifism are flawed. For example, Jesus' command to turn the other cheek (Mt 5:39) refers to a personal insult (like a slap in the face), riot to bodily harm. Indeed, even Jesus refused to turn His cheek when smitten unjustly (Jn 18:22-23). The exhortation to love our enemies does not preclude the use of force to restrain them from killing us (cp. Paul's instigation of government intervention for his protection in Ac 23).
Extracted from the Apologetics Study Bible