germany united-states
Yes, but only in some jurisdictions, and only under special circumstances.
What you describe is the retrospective application of a law that didn't exist when the action was committed. The legal term is ex post facto law..
In most jurisdictions laws can not be applied retroactively. In the U.S., the constitution forbids ex post facto law both on the federal (Article I, Section 9, Clause 3: "No Bill of Attainder or ex post facto Law shall be passed" and state (Article I, Section 10, Clause 1: "No State shall [...] pass any [...] ex post facto Law") level. In Germany, Art. 103 Abs.2 stipulates: "Eine Tat kann nur bestraft werden, wenn die Strafbarkeit gesetzlich bestimmt war, bevor die Tat begangen wurde." ("An act can only be punished if it was punishable by law before the act was committed.")1
The rationale is the very raison d'etre of codified law, namely legal certainty. An individual must ideally be able to determine whether an action is legal or not, at the time they want to commit it. The prospect of future law changes which change the legality retroactively puts the individual in a state of legal insecurity, the exact opposite of what codified law strives to achieve. This very basic consideration makes the non-retroactivity a correspondingly strong principle.
However, in Germany and probably other civil law jurisdictions, this principle finds its limits where its legal consequences are perceived as unbearably unjust. A relatively recent exception was made in the 1990s when GDR border guards were tried who killed fugitive unarmed GDR citizens. Some of them were convicted even though GDR law at that time probably made the killings legal. For example, the shooters were commended and received bonuses.
The convictions were challenged in the constitutional court which ruled:
Das strikte Rückwirkungsverbot des Art. 103 Abs. 2 GG findet seine rechtsstaatliche Rechtfertigung in der besonderen Vertrauensgrundlage, welche die Strafgesetze tragen, wenn sie von einem an die Grundrechte gebundenen demokratischen Gesetzgeber erlassen werden.
An einer solchen besonderen Vertrauensgrundlage fehlt es, wenn der Träger der Staatsmacht für den Bereich schwersten kriminellen Unrechts die Strafbarkeit durch Rechtfertigungsgründe ausschließt, indem er über die geschriebenen Normen hinaus zu solchem Unrecht auffordert, es begünstigt und so die in der Völkerrechtsgemeinschaft allgemein anerkannten Menschenrechte in schwerwiegender Weise mißachtet. Der strikte Schutz von Vertrauen durch Art. 103 Abs. 2 GG muß dann zurücktreten.
To paraphrase: The criminal code can only be trusted, and the prohibition of a retroactive application according to Art. 103 Abs. 2 GG is only applicable, if the law has been enacted by a lawful democratic government. This trust in the law is lacking when a government encourages most serious crimes and thusly flagrantly disregards generally acknowledged human rights. In such cases, the strict trust established by Art. 103 Abs. 2 GG must stand back.
In this decision, the constitutional court explicitly referred to the "Radbruch formula",
a legal theory which was first formulated in a 1946 essay by the German law professor and politician Gustav Radbruch. According to the theory, a judge who encounters a conflict between a statute and what he perceives as just, has to decide against applying the statute if – and only if – the legal concept behind the statute in question seems either "unbearably unjust" or in "deliberate disregard" of human equality before the law.
Radbruch had developed this position in 1946 under the impression of the Nazi crimes. Its application to GDR crimes were debated. While everybody agreed that the Nazis were objectively evil and unlawful, the assessment of the GDR government was less clear. The constitutional court decided that it was objectively unlawful enough that the border guards should have been able to realize that their actions were objective crimes, regardless of the positive law.
1The common exception is that a milder later law takes precedence over the stricter law valid at the time of the action. The rationale is obvious: It would be perceived as unjust to continue a criminal trial, e.g. for smoking pot a couple years ago, while outside the courtroom the prosecutor is sharing a joint with his friends.
