 Using Europe's history of state formation as his model, Herbst identifies interstate war as the factor that enabled states to effectively collect revenue and to generate a spirit of nationalism, two results that Herbst considers "crucial developments" in the formation of strong states.War increases both a leader's incentive to establish an efficient system of taxation and the population's willingness to assent to higher taxes.The existence of an external threat is also a powerful impetus for the development of a cooperative or unified state.