Many of Iran’s crimes are well-known to Americans and observers world-wide. The Tehran regime wants to build a nuclear weapon despite being a signatory to the Nuclear Non-Proliferation Treaty; it supports the brutal crackdown of Syria’s Bashar al-Assad against his own people; it is the leading state sponsor of terrorism, killing innocents from Argentina to Lebanon, Afghanistan and beyond; and it is engaged in massive domestic repression. Less recognized, however, is the legal significance of Iran’s genocidal anti-Semitic and anti-Israel rhetoric, which constitutes one of the most serious crimes under international law.

The United Nations’ Genocide Convention outlaws not only acts of genocide but “incitement” to genocide, an egregious offense whether or not genocide has yet occurred. The convention’s goal, of course, is to prevent genocide before it takes place. Tragically, warnings of impending atrocities in Rwanda were ignored by the international community. As a result, 800,000 innocent civilians were slaughtered in a genocide that could have been prevented. Iran has given the world ample warning.

A website affiliated with Iranian Supreme Leader Ali Khamenei declared in February that Iran would be justified in killing all Israeli Jews-which Tehran’s long-range missiles could accomplish in nine minutes, boasted the site. Khamenei, for his part, has called Israel a “cancerous tumor that must be removed” and declared that there is “justification to kill all the Jews and annihilate Israel, and Iran must take the helm.”

Also in February, Iranian President Mahmoud Ahmadinejad hosted Hezbollah leader Hassan Nasrallah in Tehran, as billboards in the city declared that it is every Muslim’s duty to “wipe out” Israel. “If all the Jews gathered in Israel, it would save us the trouble of going after them world-wide,” Nasrallah has said. “It is an open war until the elimination of Israel and until the death of the last Jew on the earth.”

Iranian officials’ threats are accompanied by their denial of the Holocaust and regular characterization of Jews as nonhuman or subhuman: “bloodthirsty barbarians,” “filthy bacteria,” “wild beasts,” “cattle,” “cancer,” “filthiest criminals,” “a blot,” “a stain,” “wild dogs” and the like. Similar slurs were made in Nazi Germany and Rwanda. They are the precursors to genocide.

Some argue that Tehran is unlikely to act on its threats for fear of retaliation. But Iran claims it could exterminate most of Israel’s population in a matter of minutes, so there would be little opportunity for retaliation. In any event, even if Iran’s radicals could be deterred, their incitement to genocide is still illegal under international law.

Those who incite genocide, and those who defend them, often invoke the freedom of speech. But no free-speech law condones threats of mass murder. The Nuremberg tribunal convicted and executed Nazi newspaper publisher Julius Streicher for inciting the murder of Europe’s Jews, even though he hadn’t committed murders directly. Like Khamenei and Ahmadinejad, Streicher referred to Jews as “a nation of bloodsuckers and extortionists,” adding that “The Jewish problem is not yet solved. Only when world Jewry has been annihilated will it have been solved.”

The International Criminal Tribunal for Rwanda recently convicted journalists and politicians for public statements calling for the extermination of Tutsis (even though, as with Streicher, the defendants hadn’t participated in genocidal acts themselves). The tribunal declared that the right to freedom of expression is restricted by the Genocide Convention. Since the Genocide Convention removes the traditional immunity for heads of states, former Rwandan Prime Minister Jean Kambanda was sentenced to life in prison for his incendiary speeches.

Such precedents should lead state parties to the Genocide Convention to file complaints against Iran-which is also party to the convention-before the International Court of Justice. Member states should also request that the U.N. Security Council pass a resolution condemning Iran’s incitement to genocide. They should also request that the council refer the matter to the prosecutor of the International Criminal Court, who can indict Khamenei, Ahmadinejad and their collaborators, as it has Sudanese President Omar al-Bashir. This threat of criminal prosecution should be added to existing diplomatic and economic pressures meant to deter terrorism and nuclear-weapons development by Tehran.

Silence is not a moral option when states threaten genocide-especially when they are on the verge of acquiring nuclear weapons and boast that they can bring about a holocaust in a matter of minutes.

Mr. Bernstein, a former president of Random House, was founder and chair of Human Rights Watch from 1978-98. He is now the chair of Advancing Human Rights (AHR). Mr. Cotler, a member of the Canadian parliament and emeritus professor of law at McGill University, is a former minister of justice and attorney general of Canada. Mr. Robinowitz, of counsel to Paul, Weiss LLP and former adjunct faculty member at Yale Law School, led fact-finding missions for the American Bar Association, Human Rights Watch and Helsinki Watch. He is a board member of AHR.

A version of this article appeared May 2, 2012, on page A15 in some U.S. editions of The Wall Street Journal, with the headline: Inciting Genocide Is a Crime.