A legal principle, vital within the United States legal system, prohibits the enactment of laws that retroactively punish actions that were legal when committed, or that increase the penalty for a crime after it was committed, or that alter the rules of evidence in a particular case to make conviction easier after the fact. For example, a state cannot pass a law today that makes selling lemonade without a permit in 2022 a criminal offense. Similarly, if the penalty for robbery was five years in prison at the time of the offense, a law passed after the robbery increasing the sentence to ten years cannot be applied.
This principle is enshrined in the U.S. Constitution to protect individual rights and ensure fairness within the justice system. It safeguards individuals from arbitrary changes in the law and prevents the government from unfairly targeting past conduct. Its inclusion in the foundational legal document demonstrates a commitment to the rule of law and the prevention of governmental overreach. Historically, this safeguard was crucial to prevent political persecution and ensure stability in the application of legal consequences.
