I would argue that a major cause of most (but not all) wars is religion. Religion divides people into separate groups and tells them that they are incompatible with each other. It provides justification for the killing of others and the promise of reward for martyrs.
There are numerous examples of religious wars. Some of the main ones include the Crusades, Yugoslavia, Iraq, Afghanistan, The War On Terror, The Thirty Year War, Northern Ireland and the numerous religious wars of England and France (which culminated in the Saint Bartholomew Day Massacre). A frequent justification for the British, French, Spanish, Portuguese etc, (which led to horrendous massacres) empires was so that the natives could be converted to Christianity. Religions claim they have the one truth that will save only them and all other religions are wrong and/or the work of the devil. The Torah, Bible and Koran all contain justifications for the killing of non-believers (who after all are going to Hell). They are clear that following a different religion is an unforgivable crime deserving death and destruction of entire cities (see here). 2nd Kings 10:19-27 records how every member of the religion of Baal was killed (with God’s approval). 2nd Chronicles 28:6 records the killing of 120,000 “valiant” men because they did not worship God. These are two of many examples.
Religion played a part in causing World War Two (as well as nationalism). Hitler believed he was doing God’s work (see a post on this topic), denounced atheism and regularly referred to God in his speeches. The Holocaust built upon the anti-Semitism that came from the Christian belief that Jews were guilty of murdering Jesus (see here). If there had been no religion, there would be no difference between Christians and Jews and therefore no persecution. Fascist dictatorships in Spain, Italy and Latin America were strongly supported and endorsed by the Catholic Church.
Some argue that Atheism is responsible for the crimes of Stalin, Mao and Pol Pot. However this is confusing correlation and causation. They killed people because they were brutal Communist dictators, not because of their religious beliefs (or lack of any). Collectivisation still would have failed and caused famines killing millions, regardless of the religious beliefs of the nation. For comparison, Scandinavian countries are very secular, yet are not dictatorships. (see here)
Even wars which were caused by reasons other than religion, religion was used to justify it and motivate soldiers. For example, the fact that Protestant Germany invaded Catholic Belgium was a main part of the recruitment process in Ireland during World War One, even though it was not a purely religious war.
Religion creates division between people who otherwise have little differences. It justifies hate and murder. It is responsible for numerous wars and conflicts. Along with nationalism it is the major cause of war.
(In my next post I will look at this topic in a specifically Irish context)