The accusations of Iranian meddling "illustrate what may be one of our greatest problems," said Anthony Cordesman, a former Defense Department official and military expert at the Center for Strategic and International Studies in Washington.
"We are still making arguments from authority without detail and explanation. We're making them in an America and in a world where we really don't have anything like the credibility we've had in the past."
Few doubt that Iran is seeking to extend its influence in Iraq. But the groups in Iraq that have received the most Iranian support are not those that have led attacks against U.S. forces. Instead, they are nominal U.S. allies.
The Supreme Council for Islamic Revolution in Iraq, one of the two largest parties in parliament, is believed to be the biggest beneficiary of Iranian help. The Shiite group was based in Iran during Hussein's reign, and Iran's Revolutionary Guard trained and equipped its Badr Brigade militia.
But the Supreme Council also has strong U.S. connections. Bush played host to the head of the party, Abdelaziz Hakim, at the White House in December, and administration officials have frequently cited Adel Abdul Mehdi, another party leader, as a person they would like to see as Iraq's prime minister.
The Islamic Dawa Party of Iraq's current prime minister, Nouri Maliki, also has strong ties to Iran.
Some U.S. officials have also suggested that Iran, a Shiite theocracy, has provided aid to the Sunni insurgents, who have led most of the attacks against U.S. forces. Private analysts and other U.S. officials doubt that. Evidence is stronger that the Iranians are supporting a Shiite group that has attacked U.S. forces, the Al Mahdi militia, which is loyal to radical cleric Muqtada Sadr.
Top U.S. intelligence officials have been making increasingly confident assertions about Iran.
"I've come to a much darker interpretation of Iranian actions in the past 12 to 18 months," CIA Director Michael V. Hayden said in recent congressional testimony. Previously, Tehran's priority was to maneuver for a stable Iraq dominated by its Shiite majority, but that attitude has changed, he said.
"There is a clear line of evidence that points out the Iranians want to punish the United States, hurt the United States in Iraq, tie down the United States in Iraq," he said.
