WASHINGTON — U.S. leverage over Iran all but evaporated this week, along with some of the international credibility and goodwill the Bush administration has worked to rebuild since the phantom weapons debacle in Iraq.

Washington’s turnabout on whether Iran is secretly developing a nuclear weapon instantly took President Bush’s sticks off the table, leaving him only the carrots his administration once dismissed as ineffective against an ambitious and determined adversary.

The biggest stick — a U.S. military strike on Iran in the near term — is now out of the question.

“It would be incredibly difficult to justify to either domestic or foreign audiences that any such step is even remotely necessary if there is not an active weapons effort,” said Paul Pillar, a former CIA analyst who managed the writing of previous U.S. intelligence assessments of Iran.

A military strike was always far-fetched, and the administration has been trying to apply economic and diplomatic pressure instead.

But a tougher round of United Nations sanctions against Iran may be dead on arrival now that a U.S. intelligence composite has concluded that Iran isn’t actively seeking to build a nuclear weapon.

The assessment Monday amounts to a do-over of years of U.S. insistence that Iran is driving toward a bomb that could threaten its Mideast neighbors, and it increases pressure on Bush to bargain with Iran without preconditions.

The current, mild international sanctions have done nothing to deter Iran from nuclear activities that it insists are peaceful and legitimate — enriching uranium for what it says is a civilian energy-production program.

“The Bush administration has perhaps even less credibility now in Beijing and Moscow than Iran’s clerics,” said Karim Sadjadpour, an expert on Iran at the Carnegie Endowment for International Peace. “China and Russia felt like they were sacrificing their own national interests in order to please Washington. It’s highly doubtful they will continue to do so.”