St Thomas Aquinas was a priest of the Dominican order, born in 1225. He was ordained a priest in 1250, and became Papal advisor in 1259. He is most famous as a philosopher, and a leading Catholic theologian, who came at a time when the Church was in need.
Around the time of Aquinas' birth, the teachings of Aristotle were first translated, giving rise to a group calling themselves the Averoists1 who advocated the existence of two truths, religious and philosophical. This worried the Roman Catholic church immensely, because they couldn't see a way to lead people away from this school of thinking. Fortunately for them, along came Aquinas.
Aquinas argued that the truths, far from being contradictory, were just different sides of the same truth, and complemented each other. He claimed that both sensation and thought were needed to understand the universe, and only revelation could lead to understanding the divine. He wrote many books, and after his death he was canonised for his great service to the Church.
A Just War
In respect to the rest of his work, these particular rules were not very important. However, even today the Church clings to them as a benchmark for justice. The rules themselves are in bold, the interpretations are in normal text, and the examples are in italics.
There were originally three by Aquinas;
The war must be started and controlled by the authority of state or ruler.
This means that for a war to be just, only the head of the nation can start it. It rules out civil war and rebellion.
This was not the case in the French Revolution because it was started by the people.
There must be a just cause.
This means that anyone and everyone being fought against must truly deserve it. A just cause would not include greed, revenge or pride, but it would include protection, self-defence and prevention of a worse evil.
This was not the case when Hitler invaded Poland in 1939 because the invasion was to gain land.
The war must be for good, or against evil. Law and order must always be restored.
This means that no one should ever find themselves on the side of evil for any reason, whatever the politics involved. It also means that there is a duty to return to a life of normality after the war is over.
This was not the case in the Boer war when the British immigrants revolted against the Afrikaans, since it was a thinly disguised attempt to make South Africa part of the British Empire.
Two more rules were later added by the Catholic Church, when new developments meant that some countries were much stronger than others. This meant that they would be more likely to win any war they started, and so they could start wars for more trivial purposes.
The war must be a last resort.
This means that every other option must be tried first.
This was not the case in World War I since the countries involved were prepared for war at the earliest opportunity, and trapped each other into it.
The war must be fought proportionally.
This means do not use more force than necessary or kill more civilians than necessary.
This was not the case in the bombing of Nagasaki or Hiroshima, since there were thousands of civilians killed.
Many wars in world history did not fulfil St Aquinas' conditions, and yet some are believed by others to be justified. Can there ever be a set of conditions that covers every war? Aquinas' religion influenced his thinking, and an atheist would probably come up with different rules. Or perhaps there are no suitable rules. Every war is different, and maybe every war should be considered in its own right. Beliefs about the value of peace are very personal, and no set of rules could satisfy everybody.