Fletcher v. Peck

Ex post facto law literally means "law from after the fact." Ex post facto law refers to a law that would include, as a criminal act, all violations of a law committed before the law was established. In other words, an ex post facto law would apply retroactively to citizens who committed an act before it was considered a crime. Ex post facto laws are considered unfair in most societies and are prohibited in the United States under Article I, Section 10 of the Constitution. Ex post facto laws should not be confused with retroactive laws which apply to civil law.

Only criminal legislation, not civil legislation, is affected by the constitutional prohibition of ex post facto laws. In order for a law to be considered a violation of the prohibition, it must damage the offender in some way. For example, a law that makes parole requirements more restrictive for a certain crime cannot be applied to individuals who committed the crime before the law was enacted. The use of ex post facto laws to prosecute former Nazis for war crimes, in particular the crime of "aggressive war," generated considerable debate over whether retroactive law could be applied to criminal behavior.