In the classic work Guns, Germs, and Steel: The Fates of Human Societies by Jared Diamond, the author argues that environmental circumstances rather than differences in intellect or genetics are responsible for the survival and dominance of some societies rather than others. Simple societies can evolve into more complex ones only where conditions to produce large quantities of food to nourish dense populations are present. If the flora and fauna of an area offer this possibility, then Diamond explains that "intensified food production and societal complexity stimulate each other, by autocatalysis." This refers to a chemical reaction in which the product of the reaction works as a catalyst for further reaction.
In other words, food production leads to sedentary living and higher population, which necessitates societal complexity, which in turn leads to greater food production, and so on. Societal complexity and centralization enable public works such as irrigation systems, long-distance trade, and economic specialization. Advanced food production leads to stored food surpluses. This in turn allows social stratification, or the forming of complex societies with chiefs, bureaucrats, scribes, craftsmen, and farmers.
However, Diamond explains that the amalgamation of smaller societies into larger ones does not come about peacefully and freely. Smaller societies are generally jealous and suspicious of each other. As Diamond writes in chapter 14 of the book,
Amalgamation occurs instead in either of two ways: by merger under the threat of external force, or by actual conquest.
To illustrate the amalgamation of smaller societies under the threat of external force, Diamond uses the example of the thirty or forty disparate Cherokee chiefdoms in the eighteenth century. Threatened by whites, they amalgamated into a single confederacy overseen by a decision-making council. In similar fashion, the American colonies united into a more complex society when faced with the external threat of Great Britain.
Amalgamation by conquest involves one chiefdom taking the initiative to conquer other chiefdoms and unite them by force. The example Diamond uses is the rise of the Zulu state as one chief conquered others in southeastern Africa in the eighteenth century.