Distant Drums – The unspeakable was uncovered when Washington's ambassador to Israel, warned the Iranian regime that only "a brief window" now existed to settle the impasse over its nuclear programme through democracy.    Issuing one of the most candid assessments yet made by a senior American official, he assured Israel that, if diplomacy failed, the United States was fully prepared to resort to military force to end the threat that the Jewish state faces from a nuclear armed Iran.   "At a certain stage we are going to have to decide whether diplomacy isn't going to work," he told the Israeli Bar Association.    The US Defence Secretary has announced that the US plans to bolster an Israeli anti-rocket system dubbed the 'Iron Dome’ with $70m in assistance this year. The Pentagon has already delivered $205 million in assistance in previous years.