What does the Bible say about war?

Many people make the mistake of reading what the Bible says in Exodus
20:13, "You shall not kill," and then seeking to apply this command to
war. However, the Hebrew word literally means "the intentional,
premeditated killing of another person with malice; murder." God often
ordered the Israelites to go to war with other nations (1 Samuel 15:3;
Joshua 4:13). God ordered the death penalty for numerous crimes
(Exodus 21:12, 15; 22:19; Leviticus 20:11). So, God is not against
killing in all circumstances, but only murder. War is never a good
thing, but sometimes it is a necessary thing. In a world filled with
sinful people (Romans 3:10-18), war is inevitable. Sometimes the only
way to keep sinful people from doing great harm to the innocent is by
going to war.

In the Old Testament, God ordered the Israelites to "take vengeance on
the Midianites for the Israelites" (Numbers 31:2). Deuteronomy
20:16-17 declares, "However, in the cities of the nations the LORD
your God is giving you as an inheritance, do not leave alive anything
that breathes. Completely destroy them…as the LORD your God has
commanded you." Also, 1 Samuel 15:18 says, "Go and completely destroy
those wicked people, the Amalekites; make war on them until you have
wiped them out." Obviously God is not against all war. Jesus is always
in perfect agreement with the Father (John 10:30), so we cannot argue
that war was only God's will in the Old Testament. God does not change
(Malachi 3:6; James 1:17).

Jesus' second coming will be exceedingly violent. Revelation 19:11-21
describes the ultimate war with Christ, the conquering commander who
judges and makes war "with justice" (v. 11). It's going to be bloody
(v. 13) and gory. The birds will eat the flesh of all those who oppose
Him (v. 17-18). He has no compassion upon His enemies, whom He will
conquer completely and consign to a "fiery lake of burning sulfur" (v.
20).

It is an error to say that God never supports a war. Jesus is not a
pacifist. In a world filled with evil people, sometimes war is
necessary to prevent even greater evil. If Hitler had not been
defeated by World War II, how many more millions would have been
killed? If the American Civil War had not been fought, how much longer
would African-Americans have had to suffer as slaves?

War is a terrible thing. Some wars are more "just" than others, but
war is always the result of sin (Romans 3:10-18). At the same time,
Ecclesiastes 3:8 declares, "There is…a time to love and a time to
hate, a time for war and a time for peace." In a world filled with
sin, hatred, and evil (Romans 3:10-18), war is inevitable. Christians
should not desire war, but neither are Christians to oppose the
government God has placed in authority over them (Romans 13:1-4; 1
Peter 2:17). The most important thing we can be doing in a time of war
is to be praying for godly wisdom for our leaders, praying for the
safety of our military, praying for quick resolution to conflicts, and
praying for a minimum of casualties among civilians on both sides
(Philippians 4:6-7).