Gump, James1991 0-7734-9898-2
Argues that the Zulu kingdom did not emerge as a revolutionary outburst; rather, state formation among the northern Nguni-speaking peoples of southern Africa began as much as a half-century before Shaka. The evidence suggests that this process began among lowland chiefdoms as a defensive response to the incursions of upland pastoralists. Lowland chiefdoms transformed traditional circumcision sets into multifunctional amabutho for better defense and productivity. When famine occurred in the early 1800s, major ruling houses made use of disciplined age-set regiments to compete for desirable ecological zones. The Zulu leader Shaka (ca. 1787-1828) based his expansionary program on these versatile amabutho and from them forged a centralized state.