The principle of legality is a concept of law relative to administrative or criminal conduct common in most developed nations. Generally, this rule of law requires that applicable statutes be in place prior to charging an individual with illegal activities. In other words, an act must be deemed illegal and laws put in place against it prior to being able to charge anyone with a crime or violation. Legality cannot be established, nor punishment applied, in retrospect. An act must already be illegal and punishments clearly established at the time a crime is committed in order to meet the requirements of the principle of legality.
Concepts relative to the principle of legality are found throughout history and across vastly diverse cultures. It is an accepted principle in the United States, Great Britain, Germany, and other European nations. To some cultures, in fact, such questions of legality are deemed to fall under natural law. In other words, it is universally understood and agreed that declaring an act criminal after it has been committed goes against the natural order of human behavior and existence. The assumption, under principles of legality, is that an individual cannot possibly know that an act committed today will be illegal tomorrow and thus, subject to punishment.
As a basic premise, having clearly established laws in place prior to claiming the commitment of a crime or initiating punishment gives individuals the fair opportunity to avoid wrongdoing and subsequent penalty. To declare an act criminal or illegal after the fact unjustly subjects individuals to prosecution at the whim of governmental or cultural leadership. Likewise, increasing the severity of punishment for a particular illegal act and applying such punishments retroactively is also deemed unjust and unethical. Such issues present an obvious opportunity for abuse of power to the detriment of citizens.
International human rights laws cover the concept of legality at length in such records and doctrines as the International Covenant on Civil and Political Rights, the European Convention on Human Rights, and the Universal Declaration of Human Rights. Such doctrines prevent — especially in times of political coups, war, or suspected genocide — the ability of leaders to enact new laws in an effort to charge and punish their enemies in retrospect. International agreements regarding such human rights violations provide clear sanctions and consequences for member countries who participate in such unjust practices. The overriding theory being that such an understanding of legality is necessary in order for civilized nations to conduct themselves fairly and ethically, both in their dealings with their own citizens and with individuals abroad.