The American Civil War was expected to be short and financed through additional borrowing. As Northern military setbacks mounted, it became obvious that the war would be long, bloody, and expensive. Annual federal budgets called for increased taxes to partially finance the rising costs. The Revenue Act of 1862 created innovative new tax sources and increased revenues dramatically. Tax innovations included the first federal income and inheritance taxes, the creation of the Office of Internal Revenue, and the use of adhesive revenue stamps. This article describes the political strategies and financial battles of the Civil War and postwar period, with particular focus on the Office of Internal Revenue and the introduction of new tax sources at the federal level. The major Civil War era innovation was the introduction of the federal income tax, including how to calculate taxable income and how tax collections were operationalized. The political and financial history of the income tax is a major focus of this article.