Religion
Economics
Politics

well what i can think of is M.A.I.N casues of WWI. i still think these are some of the causes of war today
Militarism
Alliances
Imperialism
Nationalism

War is the extreme manifestation of tribalism, “us” vs. “them”.

As social animals, we as individuals have a tendency to identify with some group. That group can be defined by many factors: genetics, ideology, geography, economic interests, etc. But however it’s defined, we come to see our “tribe” as an extension of ourselves. the limits of our tribe demarcate where “self” ends and “other” begins.

Whenever that tribal self comes under threat from some other tribe, we want to see our tribe come out on top (this is the drive that we ritualize in spectator sports). When we come to think that the threat actually puts the survival of our tribe in danger, we resort to war.

Natural Resources (water, oil, diamonds, gold).

Let’s not leave out Greed (Iraq’s attack on Kuwait is one example).

Entangling alliances.

Greed. Envy. Crazy old men in the sky.

Empire. War did not exist on any real scale until complex hierarchal empires were formed as a means to exploit the ever-widening “protectorate” in order to maintain cities, like Rome constantly expanding to have ever-wider inputs of grain and weapons.

Add a hefty dose of ideologies that tell people they’re the chosen people (Judaism, Christianity, Islam, Capitalism, Communism, etc.) and ruthless leaders willing to exploit that, and you have quite the recipe for war.

It may take the form of power grabs, land grabs or conquest under the cover some professed ideology (ie: the Crusades) but the bottom line to all of them is simple greed (for either power, wealth or both).

Violent language.