I somehow doubt that there was ever a single person who came up with the idea to go and fight their neighbours. However, conflict is found throughout the animal kingdom and humans are no different. The first recorded instances of war are from the Sumerians of ancient Mesopoania c. 3500 bc althought the first wars involving professional armies were by the Assyrians. This link describes the modern definition of war and when we first began fighting one another.
There is also a theory called the youth bulge model. In this model, the population has a larger percentage of young people under 30 than those over 30. This can lead to instability if there are insufficient resources to employ all the young people. In the future, worldwide resources such as food and water may become scarce, and this will probably be the source of major conflict in the future.