Iraq, with a Shiite majority that was brutally oppressed under Saddam Hussein and a history of warfare with Iran, has long looked askance at its larger Shiite neighbor.
With bellicose rhetoric against Iran rising in some quarters of the US government, and allegations by some Iraqi officials of Iranian efforts to destabilize their country, the key question becomes this: What role, if any, is Iran playing in Iraq's political future?
So far, there is almost no evidence of Iranian government involvement in the Iraqi insurgency, dominated as it is by nationalist groups and Sunni jihadists, both foreign and local, who US officials say are responsible for the suicide bombings and kidnappings that have terrorized Iraq for more than a year.
But there is growing suspicion among some Iraqi officials and political leaders that Iran is playing a role here. They claim that Sunni jihadists with ties to Al Qaeda have been allowed to travel through Iran on their way to make mischief here and also worry about Iran's ties to Shiite political groups that have their own militias.
"We know criminals and terrorists are coming through Iran into Iraq, but the question of Iranian government involvement with this is still unanswered,'' says Sabah Kadhim, spokesman for Iraq's Interior Ministry.
"I have no doubt that some of the people in the south who are working against us, perhaps some factions in the Mahdi Army, have ties to the Iranians."
Though he provided no details, Iraq's interim Defense Minster Hazem Shalam al-Khuzaei complained in an interview with the London-based, Saudi-owned newspaper Asharq al-Awsat on Tuesday that Iran was "interfering" in Iraq's affairs.
To be sure, not all Iraqi officials are as worried. On Monday, the top Iraqi diplomat in Washington said Iran is playing a supporting role. "Iran so far has had a positive role in Iraq and the Iraqi government recently asked them to cooperate even more on security,'' Rend Rahim Francke told the Associated Press. "If Iraq turns into a haven for terrorists not only Iraq but all countries in the region will be affected."
But suspicion is high inside Iraq because of a recent history of animosity between the two countries. For most of the 1980s, Iraq and Iran fought one of the 20th century's bloodiest wars. In the early 1990s, Saddam Hussein put down the Shiite uprising that followed the Gulf War with executions and torture that drove many Shiite leaders and supporters across the border into relative safety with their co- religionists in Iran.
Later, Iran poured money into outlawed dissident groups like the Supreme Council for the Islamic Revolution in Iraq (SCIRI) and the Dakwah Party to undermine Mr. Hussein, just as Iraq sponsored terrorist groups like the Mujahidin e-Khalq, dedicated to overthrowing Iran's Shiite theocracy. Since Hussein's downfall, Sciri and Dakwah have emerged as major political players.
"The Iranians are our old enemies,'' says a senior Sunni tribal leader, who asked not to be identified. "I expect they're behind a lot of our problems. They don't want a stable and prosperous Iraq."
Mr. Khadhim says Iraqi officials arrested a Libyan man last week, who had come overland from Afghanistan.
He says the man was planning attacks in the Shiite shrine city of Najaf "and it looks like he has ties to Al Qaeda." Another interior ministry official says that in early July that two Iranians were caught in an explosives-filled car, but there was no evidence that they were working for Tehran.
Analysts say it's hard to imagine Iran working closely with the jihadists, who, at least in their rhetoric and stated aims, seem to have much in common with Al Qaeda. Al Qaeda's exclusivist religious ideology sees the Shiites as apostates, and US officials say a key element of the group's strategy in Iraq has been to stir up a Sunni-Shiite civil war - something that could head off eventual Shiite dominance in Iraq and hurt Iran's interests.
Analysts say that while Iran has vital interests in Iraq, they doubt that the country's efforts will veer in a destabilizing direction here.
"I think it would be very naïve to assume that Tehran is playing no part whatsoever. They're in contact with a number of Shiite figures. Tehran is concerned, as the leading Shiite Muslim country, to see a powerful Shiite government come to power in Iraq,'' says MJ Gohel, a political scientist at the Asia Pacific Research Institute in London. "There's nothing wrong with that, given that a majority of Iraq is Shiite."
Juan Cole, a professor at Michigan University, says that while Iranian officials do worry that US success in Iraq could encourage America to invade their country, that isn't translating into direct efforts to oppose the US experiment here.
"I don't think they want instability in Iraq,'' he says. "They want stability, but with a Shiite majority and US influence curtailed. The question for them is the best way to get from here to there."
And that's something being fought out within Tehran's government between factions that want closer ties with the West and more hard-line groups most worried about US intentions.