A just war is a war which is declared for right and noble reasons and fought in a certain way. A just war is not a war that is ‘good’ as such – it is a war that Christians feel to be necessary or 'just' in the circumstances, when all other solutions have been tried and have failed. It is a necessary evil and a last resort.
Christianity is not a pacifist religion although there are pacifists in most Christian denominations. Some Christian groups, eg the Religious Society of Friends (Quakers), oppose war in all circumstances.
Most Christians would support a war if it were justified by just war standards.
The Just War theory was first developed by St Thomas Aquinas. Aquinas was one of the most influential theologians of the last 1,000 years. The theory set out conditions against which to judge whether or not a war should be waged (jus ad bellum) and if it could be justified, and how it should be waged (jus in bello).
The Just War theory, with some amendments, is still used by Christians and others today as a guide to whether or not a war can be justified.
Sometimes war may be necessary and right, even though it may not be good. In the case of a country that has been invaded by an occupying force, war may be the only way to restore justice. Pope Benedict XVI said
defending oneself and others is a duty.
When wars are fought to protect people it could be seen as an example of Jesus' teaching:
Love your neighbour as you love yourselfMatthew 22:39
Righteous anger is sometimes justified, eg when Jesus was angry about the presence of money lenders in the Temple, he is said to have:
made a whip out of cords, and drove all from the temple area, both sheep and cattle; he scattered the coins of the money changers and overturned their tablesJohn 2:15
Righteous anger is about using one's anger in a way that is thoughtful and controlled to bring about about justice or to protect the weak.