Historically many wars happened in the world. However, there was one reason common. More than 70% of wars happened due to hate.
You can take any major war - be it Crusades / Cold War / World Wars / Holocaust - all actually happened due to human hate. Even racism which we see today is child of this hate.
Since you have researched on history / social studies - I guess you are the right authority. My question is:
Why do people hate?
I would not say that wars are due to hate, but rather hate may be a byproduct of war. Leaders of nations go to war for numerous reasons. These include power and glory, for personal aggrandizement, because they believe that their ideology/religion is the right one and the other country's is wrong, to avenge perceived wrongs, because of religious, political or ethnic superstitions, or to defend themselves from some aggression.