Benjamin Netanyahu, the chairman of the Likud party who hopes to become Israel's next prime minister, warned on Thursday that a dangerous threshold will be crossed if Iran obtains nuclear weapons, and urged world leaders to make sure it does not happen. After a meeting with French President Nicolas Sarkozy, the rightwing front-runner in Israels upcoming general elections laid out his plans for peace in the Middle East and called Iran the greatest historical challenge facing the world. 12/19/08