Back when the federal Sentencing Guidelines were mandatory, there was broad agreement that an increase in a guideline after the completion of a crime could not be applied without violating the Constitution’s Ex Post Facto Clause, which protects against such after-the-fact increases in punishment. But since 2005, the government has been arguing that that rule no longer applies since the Guidelines are no longer mandatory and therefore an increase in a Guideline does not necessarily increase the sentence. The Supreme Court shut that argument down today in U.S. v Peugh, holding that sentencing a defendant based on a sentencing range that was increased after his crime was completed violates the Ex Post Facto Clause. In certain cases — especially fraud and child pornography — the decision has dramatic consequences because the applicable guidelines have been repeatedly and precipitously increased over the years. Peugh appears to harden the Second Circuit rule, established in U.S. v Ortiz, under which reversal was only required if the ex post facto violation could be shown to have affected the sentence. At sentencing, defendants need to carefully consider the guidelines in effect at the time of the crime as well as the current guidelines.