Ayatollah Mohammad Baqr al-Hakim was one of Iraq's best-known Shia Muslim figures.
Ayatollah Hakim spent 23 years in exile
He headed the Supreme Assembly for Islamic Revolution in Iraq (Sciri), one of the main Shia Muslim groups jockeying for power in post-war Iraq.
The group had, until recently, been based in Iran and owed much to the conservative clerics ruling Iran, who have funded the organisation for 20 years.
Ayatollah Hakim, 63, was imprisoned and tortured as an opposition leader in Iraq during the 1970s and finally fled to Iran in 1980. At the time in Iran, Ayatollah Ruhollah Khomeini was setting up a Shia Islamic state.
Ayatollah Hakim spent more than two decades in exile. From Iran, he not only headed Sciri but also controlled the group's armed wing, the Badr brigade, which had an estimated 10,000-15,000 fighters. His militias waged a low-level war of ambushes, sabotage, and assassinations against the regime.
He returned to Iraq in triumph in May, just after major combat operations were declared over by the US.
Thousands of Shia Muslims welcomed him back to his birthplace, the holy city of Najaf in southern Iraq.
Many Shia saw him as the best hope of reversing the suppression of their political aspirations throughout Saddam Hussein's period in power. Others, not wishing for an Iranian-style religious rule were wary of his Iranian connections.
But, despite its name and Iranian connections, Sciri said it was not pushing for an Iranian-style Islamist government in Iraq. The ayatollah, himself, advocated a modern Islamic state that rejects religious extremism and is independent of foreign powers in Iraq. He also said he favoured free elections for the country.
On his return to Iraq, he sought to play down fears about his links to the Badr brigades, stressing that he was not seeking to remake Iraq in the image of Iran's Islamic republic.
"We don't want an extremist Islam," he told a rally of supporters, adding that he sought "an Islam of independence, justice and freedom".
The US administration had long been wary of Ayatollah Hakim. Sciri boycotted the first US-sponsored meeting of Iraqi factions on 15 April.
When he returned to Najaf, there were fears that attempts would be made on his life by remnants of Saddam Hussein's regime, or by the lawless armed gangs who were said to roam the city.
Attacks on the regime
Five of Ayatollah Hakim's brothers and more than a dozen other relatives were killed during three decades of struggle against Saddam Hussein's Baath party.
In 1991, after the first Gulf War, President Bush senior encouraged Iraqis to rise up against their leader.
The opposition, including the Kurds of the north, believed this would mean the US would back a rebellion.
The Badr brigades crossed the border into southern Iraq and Shia strongholds, including the holy city of Najaf on the Euphrates, rose in revolt. Lacking US support, it was brutally suppressed.