#260..17th century
Bosh! Pish posh and bosh! I says now. What makes people willing to risk life, limb and property -- to die for a cause? In large number, over longish, at-the-time-indeterminate periods.
Nothing less than "religion". Their deep internal beliefs.
Look to that religious impulse, marshalled in a nation, in a region, in a group -- there you find by my hypothesis the proximal, the triggering and the sustaining cause of war.
It's no easy thing for us moderns to look into that aspect -- even though it is fundamental, intrinsic and without it no cause may be found but a plethora of nonsense.
What say, is the religious founding of the Viet Nam War?