As to whether, or not, war is justified in the Bible, it's important to understand that it is God who has established government within civilized society.  And He has given government the right to punish for wrongdoing, even to the point of death.¹ 

The primary duty of government is to establish law and order and to protect its citizens from those who would harm them.  Sadly, there are still governments within the world who, if given the opportunity, would not only murder their own citizens, but, also, enslave the citizens of other countries by conquest.  These evil governments need to be restrained; and quite often this is done by force when diplomacy fails to work.  All we have to do is look at history to be reminded of what happens when nothing is done in the face of evil.  Remember what Pol Pot did to the citizens of Cambodia after the Vietnam War; when millions were killed by the Khmer Rouge.  Also, remember what Saddam Hussein did to the citizens of Iraq and the countries around him.  We are all aware of what the Taliban did in Afghanistan; they brutalized their citizens and protected Al Queida which inflicts terrorism around the world. 

The worst thing a good person or country could do in the face of evil is to do nothing.²

Even though our desire is that evil governments change, history has shown that this does not always happen; sometimes they must be removed.  No civilized person wants war.  At times, however, it's thrust upon them through the evil actions of others.

Bible References:

¹ (Romans 13:1-4)

² (Proverbs 25:26)