BAQUBAH, IRAQ — If there is anywhere Iran could easily stir up trouble in Iraq, it would be in Diyala, a rugged province along the border between the two nations.
The combination of Sunni Arab militants believed to be affiliated with Al Qaeda and Shiite Muslim militiamen with ties to Iran has fueled waves of sectarian and political violence here. The province is bisected by long-traveled routes leading from Iran to Baghdad and Shiite holy cities farther south in Iraq.
But even here, evidence of Iranian involvement in Iraq's troubles is limited. U.S. troops have found mortars and antitank mines with Iranian markings dated 2006, said U.S. Army Col. David W. Sutherland, who oversees the province. But there has been little sign of more advanced weaponry crossing the border, and no Iranian agents have been found.
In his speech this month outlining the new U.S. strategy in Iraq, President Bush promised to "seek out and destroy" Iranian networks that he said were providing "advanced weaponry and training to our enemies." He is expected to strike a similar note in tonight's State of the Union speech.
For all the aggressive rhetoric, however, the Bush administration has provided scant evidence to support these claims. Nor have reporters traveling with U.S. troops seen extensive signs of Iranian involvement. During a recent sweep through a stronghold of Sunni insurgents here, a single Iranian machine gun turned up among dozens of arms caches U.S. troops uncovered. British officials have similarly accused Iran of meddling in Iraqi affairs, but say they have not found Iranian-made weapons in areas they patrol.
The lack of publicly disclosed evidence has led to questions about whether the administration is overstating its case. Some suggest Bush and his aides are pointing to Iran to deflect blame for U.S. setbacks in Iraq. Others suggest they are laying the foundation for a military strike against Iran.
Before invading Iraq, the administration warned repeatedly that Saddam Hussein was developing nuclear, chemical and biological weapons. Those statements proved wrong. The administration's charges about Iran sound uncomfortably familiar to some. "To be quite honest, I'm a little concerned that it's Iraq again," Sen. John D. Rockefeller IV, head of the Senate Intelligence Committee, said last week, referring to the administration's comments on Iran.
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.
One high-ranking intelligence official in Washington acknowledged a lack of "fidelity" in the intelligence on Iran's activities, saying reports are sometimes unclear because it is difficult to track weapons and personnel that might be flowing across the long and porous border.