At the beginning of the twentieth century the United States laid claim to an overseas empire, consolidating its victory in the Spanish-American War by adopting novel structures of colonial rule over a brace of newly acquired island territories. A set of Supreme Court decisions known collectively as the Insular Cases established the legal authorization for this undertaking. As the traditional story goes, they did so by holding that the U.S. Constitution did not "follow the flag" to the recently annexed possessions in the Pacific Ocean and the Caribbean Sea: thus unfettered, an ambitiously imperial nation could attend to the business of governing outre-mer peoples and places without undue attention to the republican niceties that obtained on native soil. This Article argues that this reading of the Insular Cases is fundamentally wrong. That the cases inaugurated an American empire cannot be denied. But they did not do so by creating a "Constitution-free zone" under U.S. sovereignty. Instead, the epochal significance of the cases lies in their careful creation of a new kind of U.S. territory: a domestic territory that could be governed temporarily, and then later, if necessary, be relinquished. This was no small matter for Americans contemplating a global demesne, since the Civil War had, not so long before, inscribed in blood the indivisibility of the Union; in the Insular Cases, the Supreme Court ensured that this costly principle would not bind America to its colonial periphery. In sum, the Insular Cases installed a doctrine of territorial deannexation in American constitutional jurisprudence, and in doing so they created the conditions of possibility for an American experiment with colonial governance. The experiment was not, finally, an unqualified success, but the Insular Cases remain good law, shaping life for more than four million U.S. citizens in Puerto Rico and the other remaining nonstate possessions, and doing service in recent cases dealing with the extraterritorial applicability of the Constitution. In view of this durable legacy, a properly revised understanding of the Insular Cases is essential.