This chapter provides an introduction to the intertwined histories of gender and war from the end of the Age of Revolutions in the early nineteenth century to the outbreak of the First World War in 1914. In the opening section, it offers a reconsideration of the notion of Europe’s post-Napoleonic century as an “era of peace” and of key concepts that historians have used to make sense of pursuits of war and military force during this time period, such as total war, imperialism, and militarism, and nationalism. It then offers a panoramic view of the major wars waged from the 1830s to the 1910s, paying special attention to the often porous and fluid boundaries between national, colonial, and imperial armed conflicts. Next, the chapter surveys the peacetime militarization of the “Western world” before the era of the two world wars, analyzing it as part of the movement of politics, society, culture, and economics in what was by the mid-nineteenth century a global age. The chapter concludes with an exploration of the intersections of war and gender and a reflection on the state of scholarship.