While many analysts argued he had used the Iraqi insurgency as a springboard to expand his operations, others said his influence was exaggerated.

Pretext for war

In the run-up to the Iraq war in February 2003, US Secretary of State Colin Powell told the United Nations that Zarqawi was an associate of Osama Bin Laden who had sought refuge in Iraq.

Intelligence reports indicated he was in Baghdad and - according to Mr Powell - this was a sure sign that Saddam Hussein was courting al-Qaeda, which, in turn, justified an attack on Iraq.

But some analysts at the time contested the claim, pointing to Zarqawi's historical rivalry with Bin Laden.

Both men rose to prominence as "Afghan Arabs" - leading foreign fighters in the "jihad" against Soviet forces in Afghanistan in the 1980s.

It was a far cry from Zarqawi's youth as a petty criminal in Jordan, remembered by those who knew him as a simple, quick-tempered and barely literate gangster.

But after the defeat of the Soviets in Afghanistan, Zarqawi went back to Jordan with a radical Islamist agenda.

Sentenced to death

He spent seven years in prison there, accused of conspiring to overthrow the monarchy and establish an Islamic caliphate.

Not long after his release under a general amnesty, he fled the country.

Jordan tried him in absentia and sentenced him to death for allegedly plotting attacks on American and Israeli tourists.

Western intelligence indicated Zarqawi had sought refuge in Europe.

German security forces later uncovered a militant cell which claimed Zarqawi was its leader.

Cell members told their German interrogators their group was "especially for Jordanians who did not want to join al-Qaeda".

According to the German intelligence report, this "conflicts with... information" from America.

Kurdish connection

The next stop on his itinerary was his old stamping ground - Afghanistan.

He is believed to have set up a training camp in the western city of Herat, near the border with Iran.

Students at his camp supposedly became experts in the manufacture and use of poison gases.

It is during this period that Zarqawi is thought to have renewed his acquaintance with al-Qaeda.

He is believed to have fled to Iraq in 2001 after a US missile strike on his Afghan base.

US officials argue that it was at al-Qaeda's behest that he moved to Iraq and established links with Ansar al-Islam - a group of Kurdish Islamists from the north of the country.

Sectarian strategy

In October 2002, Zarqawi was blamed for the assassination of US aid official Laurence Foley in Amman.

Months later, in 2003, he was named as the mastermind of a series of lethal bombings - from Casablanca in Morocco to Istanbul in Turkey.

It is in Iraq, though, that he was said to have been most active.

A letter released by the Americans in February 2004 seemed to support their claim that targeting Shias is central to Zarqawi's strategy in Iraq.

In it, Zarqawi appeared to share his plans for igniting sectarian conflict in Iraq as a means of undermining the US presence there.

Within days of the letter's release, bomb attacks on recruiting centres for the Iraqi security forces had killed nearly 100 people.

Another approach that sent shockwaves around the world was the beheadings of foreign hostages, which were posted on the internet in video footage attributed to the Tawhid and Jihad group.

Bin Laden rival?

The US military claimed to have injured Zarqawi in an assault in 2005. A statement released by al-Qaeda appeared to confirm this but said the injuries were minor.

Several men alleged to be key aides of Zarqawi have also been killed or captured - but these appeared to have had no effect on his group's ability to operate.

The US offered a $25m bounty on Zarqawi's head - the same sum they offered for Bin Laden himself.

But in the last year, he seemed to have been able to move his campaign beyond Iraq's borders again, claiming responsibility for a triple suicide bombing in the Jordanian capital Amman in November 2005, as well as other attacks.