In this case currently before the U.S. Supreme Court, petitioner Gamble's brief demonstrates that there was no dual sovereignty doctrine before the mid-19th century. At the Founding and for several decades thereafter, a prosecution by one sovereign was understood to bar a subsequent prosecution by all other sovereigns. Dual sovereignty is thus contrary to the original meaning of the Double Jeopardy Clause. Defendants today enjoy a weaker form of double jeopardy protection than they did when the Bill of Rights was ratified. But that fact only raises three further questions. First why did the Court erroneously conclude in Bartkus v. Illinois, 359 U.S. 121, 131 (1959), that the English and early American sources are “totally inconclusive” as to whether dual sovereignty existed at the Founding? Second, how, when, and why did the dual sovereignty doctrine come to exist? Third, given this history, why did the Court hold in United States v. Lanza, 260 U.S. 377 (1922), that a state prosecution does not bar a subsequent federal prosecution for the same conduct? This amicus brief answers these three questions. First, in Bartkus the Court simply misunderstood the English and early American sources. Second, dual sovereignty grew out of the intense controversy over slavery in the period immediately before the Civil War. The Court invented dual sovereignty largely to prevent free states from blocking the recapture of fugitive slaves. Third, by the time of Lanza, the dual sovereignty doctrine had been restated so often that the original meaning of the Double Jeopardy Clause had been forgotten. In Lanza, in any event, the Court was less concerned with original meaning than with rampant disregard for Prohibition. One purpose of dual sovereignty was to prevent “wet” localities from nullifying the Volstead Act. In short, dual sovereignty is an accident of history. It was not part of the constitutional design.