This model, however, is essentially a static one. It does not provide clues as to why nations’ behavior may change over time. Richard Bonney has addressed this problem in his writings on mainly the early modern states.  He has emphasized that the states’ revenue and tax collection systems, the backbone of any militarily successful nation state, have evolved over time. For example, in most European states the government became the arbiter of disputes and the defender of certain basic rights in the society by the early modern period. During the Middle Ages, the European fiscal systems were relatively backward and autarchic, with mostly predatory rulers (or roving bandits, as Mancur Olson has coined them).  In his model this would be the stage of the so-called tribute state. Next in the evolution came, respectively, the domain state (with stationary bandits, providing some public goods), the tax state (more reliance on credit and revenue collection), and finally the fiscal state (embodying more complex fiscal and political structures). A superpower like Great Britain in the nineteenth century, in fact, had to be a fiscal state to be able to dominate the world, due to all the burdens that went with an empire.