This is a brief conceptual model of “Long Wars” as socio-economic phase-transitions. I’ll draw an analogy with sandpiles. The wars are the avalanches on the slopes of the sandpile.
Wars like the Peloponnesian War and World War I elude easy explanation. Thucydides tried to explain it, but the local causes seem insignificant to the scale of the multi-decade long wars.
Instead, the wars are caused by a large number of small events because of changing economies and political ideas.