There are a number of other theories than the ones described in this website. Here is an overview of a few of them.
Historians tend to be unsure when it comes to explanations for all wars. A. J. P. Taylor, a renowned British historian (1906-1990), described armed conflicts as being like traffic incidents:
There are some conditions and situations that make them more likely but there can be no system for predicting where and when each one will occur.
(Via Wikipedia)
Psychologists have argued that human beings, especially men, are inherently violent. While society represses this violence, occasionally there is a need for an outlet outlet provided by war. They combine this theory with other notions, such as displacement where a person transfers their grievances into bias and hatred against other ethnic groups, nations, or ideologies.
Anthropologists see armed conflicts as fundamentally cultural, leant by nurture rather than nature. According to these theories the acceptance of armed conflicts is inculcated into each of us by the religious, ideological and nationalistic environment in which we live.
A popular new approach is to look at the role of information in the outbreak of armed conflicts. This theory, advanced by scholars of international relations such as Geoffrey Blainey, argues that all armed conflicts are based on a lack of information. If both sides at the outset knew the result neither would fight, the loser would merely surrender and avoid the cost in lives and infrastructure that an armed conflict would cause.
Other theories have argued that armed conflicts can be seen as an outgrowth of economic competition in a chaotic and competitive international system. That armed conflicts begin as a pursuit of new markets, of natural resources, and of wealth.