FACTBOX: Iraq's tricky relationship with Iran

(Reuters) - An Iraqi delegation is in Tehran to express concerns about Iranian backing of Shi'ite militias, Iraqi officials have said. Following is a factbox on Iraq's tricky relationship with Iran:

WHAT ARE THE ACCUSATIONS AGAINST IRAN?

Washington accuses Iran of funding, training and arming Shi'ite militias, particularly "rogue" elements of the Mehdi Army of Shi'ite cleric Moqtada al-Sadr. It says these operations are led by Iran's Qods force, a wing of the Islamic Republic's ideological Revolutionary Guards. Iran denies the charges. U.S. officials have stepped up their rhetoric against Iran since Iraqi Prime Minister Nuri al-Maliki cracked down on militias in the southern city of Basra in late March. That offensive led to the discovery in Basra, and also Baghdad, of what the U.S. military calls "very, very significant" amounts of Iranian weaponry. A senior U.S. military official told Reuters some of those arms were made in 2008.

WHY SEND A DELEGATION NOW?

The U.S. military intended to put the weapons on display but Iraq decided to confront Tehran with the evidence first. The U.S. military official said there had been a "sea change" in Baghdad's view of Iranian activity in Iraq since discovery of the new weapons, especially after Iran assured Maliki last year when he visited Tehran that it would help to stabilize Iraq.

"Basra changed it for the Iraqis. I'm not sure they believed it before. But they went to Basra and saw it first hand," said the official. He said the Iraqi delegation would bring photographs of the captured weapons with markings that showed they were Iranian origin, as well as testimony from detained militants who had received training from the Qods force.

Maliki himself has also become increasingly assertive. Some Iraqi officials say what happened in Basra was an eye-opener for Maliki -- he flew to Basra to direct the operation in late March and lived in the military headquarters there as it was pounded by rocket and mortar fire for days. At a news conference on Wednesday, Maliki declared: "I am not Iran's man in Iraq".

WHAT IS THE IRAQ-IRAN HISTORY?

Iran and Iraq fought an eight-year war in the 1980s in which hundreds of thousands were killed. But ties have improved since Sunni Arab strongman Saddam Hussein was ousted in the U.S.-led invasion and a Shi'ite-led government came to power in Baghdad. Iraq is an Arab nation while Iran's roots are Persian. Both countries have majority Shi'ite populations.

HOW INFLUENTIAL IS IRAN IN IRAQ

Iraqi officials acknowledge Iran has a lot of influence in the country, partly through its ties with Shi'ite politicians and parties that were based in Iran for years during Saddam's rule. Iran has a full-fledged embassy in Baghdad, unlike Sunni Arab states that have only low-level ties. The U.S. military says Iran's ambassador to Baghdad is a senior Qods force member.

Iran showed its influence by helping end fighting in Basra in late March between Sadr's Mehdi Army and security forces.

Iran has also never been shy in flexing its muscles in other ways. In an embarrassment for Maliki, Iran refused to allow his plane to fly across its air space in April 2007 when Maliki was going to Asia. The incident took place at a time when Iran was angry over U.S. detention of Iranians that Tehran said were diplomats but Washington said were Qods force members.

WHAT DOES IRAN WANT IN IRAQ?

U.S. military officials, diplomats and analysts believe Iran wants to keep U.S. forces bogged down and Iraq's government weak and compliant, but not to the point of total collapse.

Some Iraqi officials seem genuinely perplexed about Iran's intentions. One said Iran needed to make up its mind.