In a 2009 paper Turchin and Gavrilets argue that the emergence of complex societies is a response to the existential threat of violent warfare. They build upon the work of Karl Jaspers' conception of the Axial Age, whereby in the era 800 - 200 BC human societies undergo a revolutionary shift. The central mechanism which pushes societies into a complex stage is the intensity of the warfare. When war takes place across a metaethnic frontier, such as between agricultural and nomadic peoples, is when warfare is sufficiently intense to shift the society into a different state. Within the Axial Age, an increase in warfare intensity between the steppe peoples and the Persian and Chinese peoples forged the Achaemenid Persian empire and the Han Chinese empire, both complex societies.