This is a guest post by Walter Reich, Yitzhak Rabin Memorial Professor of International Affairs, Ethics and Human Behavior at George Washington University and a senior scholar at the Woodrow Wilson International Center for Scholars. Walter is a former director of the United States Holocaust Memorial Museum.

President Barack Obama will deliver the keynote address at Tuesday’s Holocaust Remembrance Day ceremony in the Capitol Rotunda. The theme for this year’s ceremony is “Never Again: What You Do Matters.” The theme emphasizes individual responsibility.

In what he says about this theme at the ceremony, and in what he does about it afterward, Mr. Obama should avoid platitudes. As an individual and as president - and in urgent response to the outrageously belligerent statement about the Holocaust made at a U.N. conference on Monday by President Mahmoud Ahmadinejad of Iran - he should speak not only about the past but also about the future. And in doing so, he should lead the world by example.

Until now, Holocaust remembrance has been about the past: the systematic murder by Nazi Germany of 6 million European Jews between 1939 and 1945.

Suddenly, Holocaust remembrance is also about the future. It’s about the threatened murder by Iran of nearly 6 million Israeli Jews. And, even worse, it’s about the potential murder of many millions more. The meaning of “never again” has never been as clear or as urgent.

In 1939, Adolf Hitler issued his “prophecy” that the Jews would be exterminated. And now, Mr. Ahmadinejad, even as he races to build nuclear weapons, denies that the Holocaust ever happened and threatens the elimination of Israel. In his speech Monday at the U.N.’s “anti-racism” conference in Geneva, he called the Holocaust an “ambiguous and dubious question” and a “pretext of Jewish sufferings.”

Hitler justified his animus against the Jews by accusing them of manipulating international finance and world governments. And Mr. Ahmadinejad, in his speech Monday, justified his animus against Israel, as he’d done before, by hurling the same accusations against “the Zionists.”

Mr. Ahmadinejad also argued that it’s “Zionists” - by which term he means Jews - who manipulate governments and nations. “It is time,” he said, that “the ideal of Zionism, which is the paragon of racism, be broken.” The seriousness of this threat by the bellicose leader of a country clearly rushing to amass nuclear weapons, and utterly committed to the elimination of Israel, can hardly be exaggerated.

The fact that the U.S. and eight other nations had already decided to boycott the conference, and the fact that, in the midst of Mr. Ahmadinejad’s vitriolic, Holocaust-denying and Israel-threatening speech, several dozen European diplomats walked out, won’t slow Mr. Ahamdinejad’s march toward nuclear weapons. Nor will it diminish the chance that he will actually use them. He’s been chastised before, and it hasn’t stopped him. Nor has Mr. Obama’s hope to “reach out” to him induced him to calm his bellicose rhetoric or stop spinning his centrifuges. The bellicose rhetoric, like the bellicose rhetoric of murderous leaders six and seven decades ago, sustains and justifies the rush to violence. He’s a man obsessed and determined, and is fast on his way to building the instruments of mass death.

Clearly the deadly past has become a frightening portent of a deadly future. And the Obama administration’s readiness to drop the demand that Iran suspend its nuclear program while negotiating about it could guarantee that, as the talks proceed, the centrifuges will continue to spin, the warheads will be made, the rockets will be poised, and Iran will be ready to strike.

So, on this Holocaust Remembrance Day, the past is threatening to become prologue. In Israel, sorrow is being joined by fear. And “never again,” so often an elevated but empty slogan, has become, at least for Israelis, a grim and concrete vow.

Too much is at stake - not only for Israel and its Jews but also for America and world. A nuclear exchange between Iran and Israel could kill many times 6 million, both Israelis and Iranians. And before any exchange - even if Iran only uses its nuclear weapons for blackmail - other nuclear powers, frightened by Iran, will emerge in the region. Saudi Arabia, Egypt and several others will have their own arsenals. At least the Cold War, horrible as it was in potential, could be controlled. The world created by a nuclear Iran could never be controlled. And the nuclear-tipped rockets shot off by those countries could reach well beyond the Middle East into Europe and elsewhere.

In addressing the theme of “never again,” President Obama should acknowledge not only the concrete danger of the “again” but also his responsibility, as an individual and as president, to avert it.

Mr. Obama should explain why talking to Iran is necessary. But he should also explain, as the nuclear clock ticks on, what he’d say - and what he’d do if, after a reasonable effort, it becomes apparent that Iran is only using the talks as a tactical maneuver to buy the little time still needed to build nuclear weapons.

Options are available, in the form of sharply targeted sanctions against elements in the Iranian regime, that at least have a chance of slowing, and even preventing, Iranian weaponization. Mr. Obama should make it clear that he’s ready to pursue those options, and any others he thinks might work and would be compatible with world peace - and, given the urgency, he should make clear his timetable for doing so.

He owes that to the victims of the last Holocaust. And he owes it to the potential victims, far greater in number, of the next.

This article can also be read at the Baltimore Sun.