This is a question that many people will give different answers to. Unbelievers, particularly atheists, like to say that religion causes all wars, and that Christianity is one of the most violent of them all. Some Christians would agree, we should be taking over the world. Others would claim that Christianity is so peaceful that it's not even ok to fight in self defence. What does the Bible really say about war? Should we really be violent? Should we really be excessively pacifistic? As with most issues, the answer lies between two extremes.
Sadly, war is a fact of life. In fact, historically speaking, religion as a whole has waged about 7% of all wars throughout history. This is based on a rather large sampling of the more than 1,700 wars described in the Encyclopedia of Wars by Charles Philips and Alan Axelrod. Regardless of the Christian view of war, the fact is we just don't have the influence we need to completely eliminate war. Man will wage war as he pleases, and unfortunately he pleases a lot.
In Romans 13, we are told that we must submit to the ruling authorities, because they have been established by God Himself (v1-2) for our benefit (v4). We're even explicitly told that the government "bears the sword" in order to punish evil (v4). Thus, for both the sake of conscience, and so that we don't get punished, we should submit, and even pay taxes to the government (v5-7). There is no exception listed for either bad governments, or for wartime, and so in the event that your government declares war, the principle must be applied: Obey the government.
There is an exception. We are told to obey God rather than man, and thus if the government commands you to sin, don't (Acts 5:28-29). The Nazi Prison Guard defence ("I was just following orders") will not hold up in the Divine Court.
With that being said, we are told to pray for the ruling authorities so that we may live peaceful lives in all godliness and reverence (1 Timothy 2:1-2). In war time, a Christian should pray for a swift end to the war. In peace time, a Christian should pray that war does not break out. We should pray that our rulers have the wisdom to solve international disagreements without resorting to violent conflict, and that they will have a sense of justice in order to avoid war when possible, and to justly declare/wage it when necessary.
But is it ever necessary? After all, Christianity is a famously peaceful religion. One verse I've seen quoted, particularly in the gun control debate, is Isaiah 2:4, which allegedly tells us to beat our swords into ploughshares. However, we see from verse 2 that this is about the latter days, specifically when the Lord's house shall sit on top of the mountains, and all nations flow to it. In these days, it makes sense to get rid of all weapons. Why? Because God Himself will be judging the nations and rebuking people (which is how verse 4 starts).
Interestingly, in Joel 3:9-10, we see the opposite statements. Rather than swords being beaten to ploughshares, God says "Beat your plowshares into swords". But in this case, He is not actually speaking about a good war. Rather, He is basically saying "come at me". He is challenging the rebellious nations, who sold the Jews and scattered them, and telling them that they can have all the weapons and soldiers in the world, they will not defeat Him.
This is because God Himself is a man of war (Exodus 15:3). He fights man and beast alike, and of course He wins. He is the ultimate warrior, and He is often seen commanding armies. This is particularly the case when the Jews are involved. He waged war to free them from slavery, He waged war to bring them into Israel, and He waged war to protect Israel.
So shouldn't we do the same? Shouldn't we seek to establish a Theocratic government across the world? Absolutely not. The thing about God is that He picks His own battles, and when we pick them for Him, we usually end up getting in His way. We see this in action at Jesus' arrest and crucifixion. Initially, Peter rushes to His defence, cutting off the ear of the high priest's servant. But Jesus commanded him "Put your sword in its place, for all who take the sword will perish by the sword." (Matthew 26:50-54). This is actually another thing similar to Romans 13. By fighting against the government, Peter was practically asking to be executed. It should be noted that Peter had this sword by Christ's own command (Luke 22:36-38), meaning Jesus is not necessarily against the ownership and use of weapons. And indeed, the Bible would seem to be in favor of self defence (e.g. Exodus 22:2), as well as defence of the innocent (e.g. Deuteronomy 19:10), and, from the government's perspective, capital punishment (e.g. Deuteronomy 24:16).
Jesus didn't change His mind following His arrest. Most of us are probably aware that Jesus famously said "my Kingdom is not of this world" (John 18:36), but what we tend to forget is that He followed it up with "If My kingdom were of this world, My servants would fight, so that I should not be delivered to the Jews; but now My kingdom is not from here." If. But. In other words, there would be a situation in which it would be necessary to wage war for a Theocracy. But that situation is not the situation we have, and so such a war is not necessary.
In reality, Christians are commanded to live in peace with all men so far as it depends on us (Romans 12:18). Now, when it doesn't depend on us, that's obviously a different matter. If, through no fault of your own, peace cannot be achieved, this verse at least implies that violence is a valid answer.
But what about Jesus' command to turn the other cheek (Matthew 5:39)? In context, this is limited in its application. It is about being humble and merciful in the face of an oppressor. It's one thing to make your enemy look like the jerk he is by not hitting him back, it's another thing entirely to let him seriously harm you or a loved one. It's certainly not about capitulating to an aggressive enemy, foreign or domestic. As previously stated, the government exists for the very purpose of bearing the sword. The government is intended to be a terror to evil doers and security to the good. Basically, a government's first priority should be the peace and security of its dominion.
The long and short of it is that unless a Christian actually holds a relevant position in government, they need not concern themselves with the Biblical view on war. The government has their job, but as Christians, we have ours. And we are in a war. But the Bible tells us that this is not a war of flesh and blood (Ephesians 6:12). Rather, our enemies are spiritual. Therefore, we do not wage this particular war with physical weapons, but with spiritual ones (2 Corinthians 10:3-4). We don't destroy buildings, we destroy arguments (v5). As the old saying goes, "the pen is mightier than the sword".
And when the pen fails? Jesus doesn't command us to take up the sword anyway. Instead, He says "And whoever will not receive you nor hear your words, when you depart from that house or city, shake off the dust from your feet." (Matthew 10:14, cf. Luke 9:5 and Mark 6:11). When people don't receive the Gospel, it is not our job to make them. Our only job is to offer it to them. When they reject it, Jesus effectively says "don't worry about it. I'll deal with them on Judgement Day".
There are two kinds of war: Physical and spiritual. When it comes to physical wars, Christians are to submit to the government, as always, but only so far as they also submit to the Lord. The government will give account for any unjust actions they take. As for the spiritual war, it is the Lord's battle. It is not for us to take up the sword, and we certainly need not seek to establish His Kingdom on the Earth. He's quite capable of taking that for Himself when He's ready. For now, our marching orders are to preach the Gospel as peacefully as possible. We need neither be warriors or pacifists, but rather behave wisely, recognising when violence is necessary, but defaulting towards peace. Just beware: We are accountable to a God of war, and a God of peace. Seek to please Him, and you will be rewarded.