Puerto Rico may have its own constitution, its own government, its own laws, and its own courts—but it is not a true sovereign under the Fifth Amendment’s Double Jeopardy Clause.

If that sentence confuses you, that’s because it’s confusing. It is also the Supreme Court’s holding in Puerto Rico v. Sanchez Valle, a blockbuster case that carried the potential to clarify Puerto Rican autonomy and the scope of double jeopardy. But the court resolved neither of these issues in Sanchez Valle on Thursday, preferring instead to decide the case on narrow grounds. The result is an opinion that leaves the biggest questions about Puerto Rico’s future for another day.

Sanchez Valle involved a multilayered constitutional conundrum: Is Puerto Rico sovereign enough to prosecute criminals who have already been successfully prosecuted by the federal government? You might expect the answer to be an obvious no, because the Fifth Amendment’s Double Jeopardy Clause states that no person “may be subject for the same offence to be twice put in jeopardy of life or limb.” But the Supreme Court has carved out an exception to this guarantee called the “separate sovereigns” doctrine. Under this rule, both states and the federal government can try a defendant for the same crime—states under state law, the feds under federal law. The question in Sanchez Valle is whether Puerto Rico, a United States territory, is sovereign enough to squeeze into this double jeopardy exception.

By a 6–2 vote, the court said no. In her majority opinion, Justice Elena Kagan writes that the answer to this question hinges on the “furthest-back source of prosecutorial power.” Here is her typically cogent explanation of that inquiry:

If two entities derive their power to punish from wholly independent sources (imagine here a pair of parallel lines), then they may bring successive prosecutions. Conversely, if those entities draw their power from the same ultimate source (imagine now two lines emerging from a common point, even if later diverging), then they may not.

States, Kagan explains, are the “ultimate source” of their own prosecutorial power under the 10th Amendment. But as a territory, Puerto Rico’s true source of prosecutorial authority is the United States Congress—the federal government. After all, Kagan writes, Congress ruled the island for decades before turning over power to a territorial government. And even then, Congress gave itself the authority to approve the Puerto Rican Constitution before it went into effect. Moreover, Congress technically retained the power to veto Puerto Rican laws, even though it has never exercised it. Taken together, Kagan concludes, these facts compel the conclusion that, for double jeopardy purposes, Puerto Rico is not a separate sovereign from the federal government.

Justice Stephen Breyer’s dissent, joined only by Justice Sonia Sotomayor, questions the wisdom of Kagan’s “ultimate source of power” inquiry. “We do not trace Puerto Rico’s source of power back to Spain or to Rome or to Justinian,” Breyer writes, “nor do we trace the Federal Government’s source of power back to the English Parliament or to William the Conqueror or to King Arthur.” Further, states are only admitted to the union “at the invitation of Congress.” That means the federal government is, under Kagan’s test, “in an important sense the ‘source’ of these States’ legisla­tive powers.” Should they lose their sovereign status under the Double Jeopardy Clause, too?

Breyer proposes an alternative analysis, examining “the broader context of Puerto Rico’s history” to decide whether the territory “gained sufficient sovereign authority to become the ‘source’ of power behind its own criminal laws.” He notes that in granting self-rule to the territory, Congress intended “a significant change in the nature of Puerto Rico’s political status … taking a significant step in the direction of change by granting Puerto Rico a special status carrying with it considerable autonomy.” As a result, the island should today be considered a separate sovereign for double jeopardy purposes.

Always the history buff, Breyer assembles heaps of evidence from the historical record to support his claim. When Congress permitted Puerto Rico “to convene a constitutional convention and to write a constitution” in 1950, it stated its desire to grant “the right of self-government [to] the people of Puerto Rico.” The result of this convention was a constitution that explicitly declares: “political power emanates from the people and shall be exercised in accordance with their will.” Congress, after making minor modifications, ratified this constitution. At the time, President Harry Truman wrote that the new constitution “vest[s] in the people of Puerto Rico” complete “authority and responsibility for local self-government.” The United States then told the United Nations that the island had achieved “the full measure of self-government,” which the U.N. General Assembly accepted as true. Since then, the Puerto Rican Supreme Court “has consistently held, over a period of more than 50 years, that Puerto Rico’s people (and not Congress) are the ‘source’ of Puerto Rico’s local criminal laws.”

At bottom, the central dispute in Sanchez Valle is one between abstract formalism and fact-based analysis. Kagan and Breyer have similar temperaments and ideologies, but Sanchez Valle reveals a fascinating fault line between the two justices’ judicial philosophies. Kagan locates the original source of Puerto Rico’s power on paper and calls it a day; Breyer explores “the practices, actions, statements, and attitudes” of both Congress and Puerto Rico with regard to the island’s sovereignty.

I think Breyer has the winning argument. What Kagan calls “Puerto Rico’s transformative constitutional moment” clearly severed Congress’ practical control over the island. And while Congress alleged to retain veto authority over Puerto Rican laws, it has not once exercised this power. Finally, Congress’ description of Puerto Rican sovereignty to the U.N. clinches Breyer’s case. In the 1950s, the U.S. government told the U.N. that Puerto Rico would theretofore have “freedom from control or interference by the Congress in respect to internal government and administration.” Courts should hold the U.S. government to that promise.

Ultimately, both Kagan and Breyer agreed that the court must leave bigger questions about Puerto Rican sovereignty—like Congress’ ability to deny the island debt-relief and deprive its residents access to federal benefits—for another day. And speaking of bigger constitutional questions, Justice Ruth Bader Ginsburg wrote a brief concurrence, joined by Justice Clarence Thomas, arguing that the entire doctrine of separate sovereigns under the Double Jeopardy Clause may be incorrect—essentially inviting a challenge to the notion that states and the feds may try a defendant for the same crime. Quite frankly, that may be bigger news than the decision’s central holding, which is strictly limited to one form of sovereignty in one territory. Sanchez Valle is a tremor that could presage a coming constitutional earthquake.