This paper argues that the efficiency distribution of players in a game determines how aggressively these players interact.We formalize the idea of balance of power: players fight very inefficient players but play softly versus equally (or more) efficient players.This theory of conduct predicts that entry by new firms leads to a less aggressive outcome if it creates a balance of power. A balance of power is created if more players get technologies that are close to the most efficient technology.Using a related argument, we show that an increase in entry costs can lead to more aggressive outcomes.