How do competitors react to each other's price-promotion and advertising attacks? What are the reasons for the observed reaction behavior? We answer these questions by performing a large-scale empirical study on the short-run and long-run reactions to promotion and advertising shocks in over 400 consumer product categories over a four-year time span. Our results clearly show that the most predominant form of competitive response is passive in nature. When a reaction does occur, it is usually retaliatory in the same instrument, i.e., promotion attacks are countered with promotions, and advertising attacks are countered with advertising. There are very few long-run consequences of any type of reaction behavior. By linking reaction behavior to both cross- and own-effectiveness, we further demonstrate that passive behavior is often a sound strategy, while firms that do opt to retaliate often use ineffective instruments, resulting in 'spoiled arms.' Accommodating behavior is observed in only a minority of cases, and often results in a missed sales opportunity when promotional support is reduced. The ultimate impact of most promotion and advertising campaigns depends primarily on the nature of consumer response, not the vigilance of competitors.