Periods of world-wide war and catastrophe, Strifes are violent parts of the formative history of the world. Typically these Strifes have occurred when those who worship and serve the Antiarchs attempt to overthrow the cities and territories of those who worship the Primarchs. Historically those who serve the Primarchs- humans, elves, eladrin, dwarves, and halflings have held the most fertile and successful lands on the continents, and so each Strife tends to be characterized by attempts by the other races- orcs, goblins, kobolds, etc. to gain control of those resources.
After every strife, at first after a hundred years or so, but scarcely a few years after the most recent Strife, there follows a long period of Restoration, where the lands and settlements previously seized by the Antiarch worshippers are retaken and the races of the Primarchs re-establish their dominance.