P became a territory of the United States in 1898, as a result of the Spanish-American War. The treaty concluding that conflict ceded the island, then a Spanish colony, to the United States. Over time, Congress granted P autonomy. A federal statute passed in 1917, in addition to giving the island’s inhabitants U. S. citizenship, replaced the upper house of the legislature with a popularly elected Senate. Three years later, Congress enabled P to embark on the project of constitutional self-governance. The Puerto Rico Constitution created a new political entity, the Commonwealth of Puerto Rico. Valle (D) and Vázquez (D) each sold a gun to an undercover police officer. P prosecutors indicted them for selling a firearm without a permit in violation of the Puerto Rico Arms Act of 2000. Federal grand juries indicted Ds, based on the same transactions, for violations of analogous U. S. gun-trafficking statutes. Each D moved to dismiss the pending P charges on double jeopardy grounds. The trial court dismissed the charges. The Puerto Rico Court of Appeals reversed those decisions. The Supreme Court of Puerto Rico granted review and held that P’s gun sale prosecutions violated the Double Jeopardy Clause. The Supreme Court granted certiorari.