The aim of strategy is to master a market environment by understanding and anticipating the actions of other economic agents, especially competitors. A firm that has some sort of competitive advantage--privileged access to customers, for instance--will have relatively few competitors to contend with, because potential competitors without an advantage, if they have their wits about them, will stay away. Thus, competitive advantages are actually barriers to entry and vice versa. In markets that are exposed, by contrast, competition is intense. If the incumbents have even brief success in earning greater than normal returns on investments, new entrants will swarm in to grab a share of the profits. Sooner or later, the additional competition will push returns as far down as the firms' costs of capital. For firms operating in such markets, the only choice is to forget about strategy and run the business as efficiently as possible. Barriers to entry are easier to maintain in a competitive arena that is "local," either in the geographic sense or in the sense of being limited to one product or a handful of related ones. The two most powerful competitive advantages—customer captivity and economies of scale—are more achievable and sustainable in circumscribed markets of this kind. Their opposites are the open markets and host of rivals that are features of globalization. Companies entering such markets risk frittering away the advantages they secured on smaller playing fields. If a company wants to grow but still obtain superior returns, the authors argue, the best strategy is to dominate a series of discrete but preferably contiguous markets and then expand only at their edges. Wal-Mart's diminishing margins over the past 15 years are strong evidence of the danger of proceeding otherwise.