German Chancellor Angela Merkel insisted Saturday that Iran cannot be allowed to develop nuclear weapons and said Germany's Nazi past meant it could never tolerate Iranian President Mahmoud Ahmadinejad's comments about Israel and the Holocaust. "A president that questions Israel's right to exist, a president that denies the Holocaust cannot think that Germany has even the slightest degree of tolerance. We have learned from our history," she told an international security conference. She said the Nazi experience showed the need to act to act early swiftly to tackle such extremism, but stressed the Iran should be confronted with diplomatic, not military means. "Looking back the to German history in the early 1930s when National Socialism was on the rise there were many outside Germany who said it is only rhetoric - don't get excited," Merkel said. "There were times when people could have reacted differently, and in my point of view, Germany is obliged to do something at the early stages."