Blakely v. Washington, 542 U.S. 296 (2004), held that, in the context of mandatory sentencing guidelines under state law, the Sixth Amendment right to a jury trial prohibited judges from enhancing criminal sentences based on facts other than those decided by the jury or admitted by the defendant. The landmark nature of the case (for good or ill) was alluded to by Justice Sandra Day O'Connor, who "described the Court's decision as a 'Number 10 earthquake.'"
Read more about Blakely V. Washington: Background of The Case, Majority Opinion, Dissenting Opinions, Effect On Subsequent Jurisprudence, Subsequent Developments