War is simultaneously a destructive force and a powerful engine of political and social reform. In the early modern era, when colonization of the Americas stimulated the development of European overseas empires and the parallel process of state formation, warfare and imperial reform came to be tightly linked and closely related processes. From 1689 until 1815, warfare among the principal European colonizing powers became an endemic condition, and wars increasingly spilled over into American theaters. Warfare prompted imperial reforms; those reforms, in turn, prompted further conflicts. Between 1689 and 1815, imperial competition, overseas warfare, and reform unfolded in three long eras. In the first, which extended through the first half of the eighteenth century, European powers developed a sharpened sense of commercial and territorial rivalry in the Americas. Commercial rivalries heightened Spain’s efforts to defend its maritime interests, while territorial competition energized activities in borderlands regions and led to new patterns of alliance with Native American groups. In the second half of the eighteenth century, the second era of imperial conflict and reform unfolded.