Walter is presenting part of a book about rebel groups and how competition affects their behavior during civil wars.  It starts from the observation that rebel groups often act quite differently from each other even when they are pursuing similar goals.  She argues that the structure of the competitive environment in which rebel groups operate explains much about their behavior during civil war.  The more competition rebel groups face from rival factions or potential factions, the more attention they are likely to pay to the needs of the local population and the better and more benevolently they will govern.  In addition, the greater the competition, the more likely rebel groups are to pursue strategies designed to deter and eliminate rivals.  The particular characteristics of the competitive environment, therefore, could influence the behavior of rebel groups in important ways and could help explain the variation in rebel behavior across conflicts and over time.