holy wars

   the expansion into the Middle East in the Middle Ages by western adventurers

   We know them better as the Crusades.

   Although the pretext, and motive for some, was religious, a major cause for the aggression and attempted conquest was the pressure on resources caused by the rising population in western Europe prior to the fortuitous onset of the Black Death. After humanity had been culled for a century, the problem was starting to recur when the Age of Discovery revealed softer victims in the Americas, Africa, and the East. As what goes round comes round, parts of the world now experience a jihad.