The visual arts played a pivotal role in three wars that shaped U.S. history between 1861 and the turn of the century: the Civil War, the Black Hills War, and the War of 1898. This dissertation traces the historical and visual imbrications of campaigns for U.S. empire in its consideration of conflict imagery across these military engagements. Three chapter-length case studies enable comparative analysis, which in turn helps to consolidate diverse visualizations of warfare into a coherent, revelatory, and nuanced corpus. Importantly, this project addresses the challenge of treating the field of American art history as hemispheric in its survey of varied but interrelated theaters of conflict. Select battle paintings by Peter Rothermel, Mathó Nážiŋ (Standing Bear), Edgar Paxson, Winslow Homer, and Armando García Menocal form the fulcrum of each chapter and are interpreted as embedded in, rather than separate from, a profoundly innovative moment in visual culture. This project puts forth a central argument: as U.S. war pictures mediated ongoing civil, colonial, and imperial violence through new pictorial forms in the late nineteenth century, representational truth claims became more pronounced. Amidst escalating associations between art and testimony, artists presented their war imagery as the product of “witness work”—a term I introduce and define as the range of artistic and discursive efforts to appropriate validity for iconic but mythologized events. Chapter one addresses Pickett’s Charge at the Battle of Gettysburg, chapter two explores the Battle of the Little Bighorn, and chapter three discusses the Battles of Santiago and San Juan Hill in Cuba. The performative witness process undergirding depictions of these historical events altered public understanding of them—obscuring the contribution of African Americans to the Gettysburg campaign, the right of Indigenous communities to defend their families and homelands against settler invasion, and the role of Cubans in their own revolution. Ultimately, Battle Grounds offers new, cross-cultural pathways for understanding the roles of art and witness during a period of immense turbulence throughout the Americas.