Although the Supreme Court is expected to wrap up its term at the end of the month, on Monday the Court declined to hand down any of the blockbuster civil-rights rulings still pending. It did, however, rule in Peugh v. United States, an important opinion that protected a vital democratic value: the prohibition against retroactive punishments.

The key question in Peugh involves the application of Article I, Section 9 of the Constitution, which mandates that "No ... ex post facto Law shall be passed." This prohibition reflects longstanding common-law principles central to the rule of law. For a punishment to be lawful rather than arbitrary, it must be clear that the action for which one is being punished was illegal at the time of the offense. Permitting retroactive punishments would give the state the unlimited right to take the life, liberty, and/or property of any person state actors don't like.

The ban on retroactively criminalizing criminal activity applies to sentencing as well. If an offense could carry no more than a maximum fine of $50 at the time of the offense but changed to life in prison after the fact, this also would amount to an arbitrary action.

Where the background to Peugh gets complicated is with another attempt to remedy arbitrariness in the criminal-justice system: the federal sentencing guidelines. Another way of undermining the rule of law is to give different people substantially different punishments for the same offense without adequate justification (such as prior offenses). In the mid-'80s, Congress attempted to rationalize the sentencing system by limiting judicial discretion and creating sentencing ranges that attempted to ensure that similar crimes would receive similar punishments. It delegated the task of developing a comprehensive system to the United States Sentencing Commission, whose guidelines were upheld (over a dissent by Justice Scalia, who likened the delegation to "a sort of junior-varsity Congress") by the Supreme Court in 1989.

The Supreme Court, however, threw a wrinkle into the new system in 2000. In Apprendi v. New Jersey, a coalition of the Court's three most liberal members and Justices Scalia and Thomas held that a defendant could not be sentenced based on facts found by a judge alone-the only facts that could be used to determine a sentence under the guidelines were those that had either been found by a jury or adduced in a plea agreement. In response to Apprendi and its progeny, Justice Breyer (the justice most strongly committed to the value of the guidelines) cobbled together a majority holding that the guidelines were merely "advisory" rather than mandatory. Federal judges generally followed the guidelines but were not technically required to, giving them more flexibility.

Which brings us back to yesterday's case. Marvin Peugh was convicted of bank fraud that had been committed in 1999 and 2000. When he was sentenced in 2009, however, the sentencing guidelines had changed, making the range of penalties substantially harsher. When the crimes were committed, the guidelines suggested a sentence of approximately three years. By the time of his sentencing, the range had increased to 70 to 87 months. Peugh was sentenced to 70 months in prison, 33 months longer than the maximum sentence under the guidelines in effect at the time he committed the crime.

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If the sentence range was mandatory, there would be no question that giving Peugh a significantly higher sentence than he would have received at the time his crime was committed would violate the Ex Post Facto clause. The question the Court had to answer yesterday, however, was whether applying guidelines that are merely "advisory" violates Article I. Both the District Court and the Seventh Circuit Court of appeals held that it did not; in theory, Peugh could have received the same punishment in 2000, and hence the punishment was not retroactive. Justice Thomas, writing for the four dissenters, echoed this reasoning. "To the extent that the amended Guidelines create a risk that a defendant might receive a harsher punishment," Thomas argued, "that risk results from the Guidelines' persuasive force, not any legal effect."

A majority of the Supreme Court, with Justice Sotomayor speaking for the Court's three other Democratic appointees and Justice Kennedy, reversed the lower courts and found that Peugh's sentence was unconstitutional. Based on the standard previously established by the Court's precedents, a different set of guidelines violates the Ex Post Facto clause if there is a "significant risk" that the new guidelines would lead to a higher sentence. Based on both theory and practice, Sotomayor convincingly argued that the risk was present. The guidelines, even if technically only "advisory," indirectly restrict the discretion of judges by making sentences that fall within the guidelines less likely to be overturned on appeal. Federal judges generally issue sentences that fall within the guidelines. That risk is enough to make the harsher punishment retroactive and hence unconstitutional.

The existence of the Federal Sentencing Guidelines and the ongoing adjustments made to punishments are both salutary. But given the influence of these guidelines, it is important that they do not lead to punishments harsher than defendants would have received at the time they committed an offense. Justice Sotomayor's opinion correctly upholds a core principle of the rule of law by evaluating the guidelines as they work in practice and guarding against retroactive punishments.