US President Barack Obama urged the world to combat anti-Semitism and Holocaust denial but did not refer to the threat of Iran, even as several other speakers at Thursday's national Holocaust memorial ceremony warned about the danger Teheran poses to the Jewish state."How do we ensure that 'never again' isn't an empty slogan, or merely an aspiration, but also a call to action? I believe we start by doing what we are doing today - by bearing witness, by fighting the silence that is evil's greatest co-conspirator," Obama told the congressmen, diplomats and Jewish leaders gathered in the Capitol Rotunda.He added that such tribute wasn't "the end of our obligation; it's just the beginning," since "today and every day we have an opportunity as well as an obligation to confront these scourges" of anti-Semitism, Holocaust denial, racism and other forms of intolerance.But Obama largely avoided any allusions to current political crises or their solutions, instead concluding by saying, "May each of us renew our resolve to do what must be done. And may we strive each day, both individually and as a nation, to be among the righteous."In contrast, Israeli Ambassador Sallai Meridor, referred to the display on why Auschwitz wasn't bombed in the US Holocaust Museum, which sponsored Thursday's National Commemoration of the Days of Remembrance, before asking: "When a regime is again endangering world peace, terrorizing its neighbors and threatening to destroy the Jewish people, how will we meet this challenge, before it is too late?"And US Holocaust Memorial Council Vice Chairman Joel Geiderman said pointedly, "In the names of the victims, I call on the assembled leaders and the rest of the world to ensure that no country that threatens such destruction will ever obtain the means to achieve it. Nuclear weapons in the hands of aggressor fanatics can't be allowed."Elie Wiesel, the Nobel Peace Prize-winning Holocaust survivor, labeled Iranian President Mahmoud Ahmadinejad the world's "number one Holocaust denier" and criticized the sponsors of the UN anti-racism conference in Geneva who gave him a platform this week.Wiesel, in expressing his disappointment at the event, and the implication that the world had not learned the lessons of the Holocaust sufficiently, praised Obama for not participating."Thank you Mr. President for deciding that America should boycott that gathering," he said to applause.