Fool me once, shame on you. Fool me twice, shame on me. That adage is uppermost in the minds of critics of U.S. foreign policy amid warnings by the Bush administration that Iran is actively working to destabilize Iraq. U.S. credibility has been badly damaged by the mishandling of intelligence prior to the invasion of Iraq and the new allegations have raised fears that Washington is again preparing the ground for military action. It is a sad commentary on America's international image that the Bush administration's denials are subject to such harsh scrutiny and that there is so little faith in Washington.

The United States has claimed for some time that Iranian forces were helping insurgents in Iraq fighting the Baghdad government and the U.S.-led coalition. Last week, military sources, in an off the record briefing, presented evidence behind the claim. Proof included Iranian-made weapons and copies of fake identity cards on detainees alleged to be members of the Quds Force, a special unit of Iran's Revolutionary Guards. It was then claimed that the unit reported directly to Iran's supreme leader, Ayatollah Ali Khameni. In other words, the U.S. was claiming that the highest levels of the Tehran government are implicated in attacks on U.S. forces.

There is little doubt that Iranian weapons are being used in Iraq. Iran fought a bloody eight-year war with Baghdad and has ever since provided support for co-religionists in Iran battling the Saddam Hussein regime. That is likely to include weapons, training and funding. Iran has good reasons to meddle in Iraq. It would like its Shiite allies to have more influence in Iraqi politics. It wants the Baghdad government to remain weak so that it cannot threaten Iran and Tehran is the major power in the region, and it would like to see the U.S. tied down and embarrassed. But it is another thing altogether to assert that the supreme leadership in Iran has sanctioned direct attacks on U.S. forces -- a move akin to a declaration of war.