For 30 years, the legal regime that emerged from the Civil War and Reconstruction was a “constitutional lion” barring the path of expansion. It made citizenship, rights, and eventual statehood prerequisites to any annexation. Before ratification of the Fourteenth Amendment, a U.S. annexation occurred at least every fifteen years; in early 1898, thirty years had passed since Alaska was annexed, in 1867. Rather than admit ostensible racial inferiors into the polity as citizens with rights, U.S. officials declined to acquire new lands. Instead, they projected power abroad, consolidated power at home, and subordinated peoples on both sides of the border. Eschewing annexation avoided direct challenges to constitutional constraints on empire. But officials’ actions circumscribed relevant doctrines, paving the way for attempts to slay the lion by century’s end. With the U.S. rout of Spain in a brief war in 1898, Puerto Rico became a potential testing ground for new approaches to empire. Liberals there had extracted wartime reforms from Spain. Whether those reforms would continue depended on whether the U.S. Constitution would prevent annexation of Puerto Rico or ensure that it proceeded along generous lines.