Taking your question literally (“Did the Nuremberg Trials violate article 7 of the European Convention on Human Rights?”) the answer is no.

The Nuremburg trials occurred in 1945-46, but the European Convention on Human Rights wasn’t adopted until 1950 and entered into force only in 1953. So the Nuremberg trials couldn’t violate a Convention that did not exist.

If the question is would the Nuremberg trials have violated article 7, that is a more of a legal question than a historical one. But the answer also seems to be “no”.

The Nuremberg Trials and Questions of Legitimacy

The Nuremberg trials were controversial at the time and some argued that the Nuremberg tribunal should not be able to convict defendants of crimes that did not exist at the time the acts were committed.

For example, United States District Court Judge Charles Wyzanski wrote an article for The Atlantic in April 1946 entitled Nuremberg: A Fair Trial? A Dangerous Precedent which begins:

The Nuremberg War Trial has a strong claim to be considered the most significant as well as the most debatable event since the conclusion of hostilities. To those who support the trial it promises the first effective recognition of a world law for the punishment of malefactors who start wars or conduct them in bestial fashion. To the adverse critics the trial appears in many aspects a negation of principles which they regard as the heart of any system of justice under law.

Judge Wyzanski argued that some of the charges did not rely on the creation of new law. The murder of German minorities could be prosecuted because they violated “internal laws” that existed at the time the acts were committed. This is analogous to Art. 7 § 2 But he was concerned that some of the charges were ex post facto.

The feeling against a law evolved after the commission of an offense is deeply rooted. Demosthenes and Cicero knew the evil of retroactive laws: philosophers as diverse as Hobbes and Locke declared their hostility to it; and virtually every constitutional government has some prohibition of ex post facto legislation, often in the very words of Magna Carta, or Article I of the United States Constitution, or Article 8 of the French Declaration of Rights. The antagonism to ex post facto laws is not based on a lawyer's prejudice encased in a Latin maxim. It rests on the political truth that if a law can be created after an offense, then power is to that extent absolute and arbitrary. To allow retroactive legislation is to disparage the principle of constitutional limitation. It is to abandon what is usually regarded as one of the essential values at the core of our democratic faith.

For example, he questioned whether “crimes against peace” and “conspiracy” to commit crimes against peace and humanity were not crimes prior to the Nuremberg trials.

Judge Wyzanski’s article elicited comments from many esteemed legal minds and those comments caused Judge Wyzanski to change his mind and conclude that they could be found guilty of the crime of aggressive war:

On further reflection I have come to the view that the points stated in the last paragraph are not conclusive. I am now persuaded that in the formative period of international law it is just for a representative group of power retroactively to label as criminal, conduct which, when it occurred, was universally regarded as a serious violation of generally accepted international standards and treaties.

Although Judge Wyzanski was an American judge, I think his thoughts on the issue are relevant because (1) the US was obviously heavily involved in the Nuremberg trials, and (2) Article 7 enshrines a concept that also appears in US law – the prohibition against ex post facto prosecutions.

The European Convention of Human Rights

The Convention was passed in 1950 and contains Article 7:

1. No one shall be held guilty of any criminal offence on account of any act or omission which did not constitute a criminal offence under national or international law at the time when it was committed. Nor shall a heavier penalty be imposed than the one that was applicable at the time the criminal offence was committed.
2. This Article shall not prejudice the trial and punishment of any person for any act or omission which, at the time when it was committed, was criminal according to the general principles of law recognised by civilised nations.

Article 7 relates to ex post facto prosecution, which is the same concern raised about the Nuremberg trials. But the language in § 2 is interesting because it still allows prosecutions for acts that were “criminal according to the general principles of law recognised by civilised nations.” This sounds a lot like what the Nuremberg prosecutors argued – the actions of the Nazis were illegal based on general principles enshrined in various conventions and codes of conduct.

A recent case sheds light on what this language in Article 7 means. In 2010, the European Court of Human Rights issued an opinion in Kononov v. Latvia which was a prosecution based on events from 1944. Kononov, a Soviet partisan, was charged with being part of a group that donned Wehrmacht uniforms, entered a Latvian village and killed and wounded citizens in reprisal for alleged cooperation with the Germans.

The ECHR held that Kononov’s conviction did not violate Article 7 because

the applicant’s crimes constituted crimes under the “general principles of law recognised by civilised nations” within the meaning of Article 7 § 2 of the Convention. This provision was also drafted to eliminate any doubt about the validity of the post-Second World War prosecutions by the IMTs and, since subsequent international and national practice had confirmed the universal validity of the IMTs and their principles, that role of Article 7 § 2 was now defunct. Whether such “general principles” were a primary or secondary source of international law, they were derived from national systems to fill gaps in positive and customary international law.

So Article 7 of the Convention has been interpreted as endorsing the legal theory underlying the Nuremberg trials.