Who was Islamic State leader Abu Bakr al-Baghdadi?

The US put up a $US25 million reward for his capture, the same amount as it had offered for al-Qaeda leader Osama bin Laden and his successor Ayman al-Zawahri.

US air strikes have killed most of Baghdadi's top lieutenants and thousands of his fighters have also been killed or captured.

Who was Al-Baghdadi?

Al-Baghdadi was born Ibrahim Awad al-Samarrai in 1971 in Tobchi, a poor area near the town of Samarra, north of the Iraqi capital Baghdad, whose name he took.

His family included preachers from the ultra-conservative Salafi school of Sunni Islam, which sees many other branches of the faith as heretical and other religions as anathema.

He joined the Salafi jihadist insurgency in 2003, the year of the US-led invasion of Iraq, and was captured by the Americans. They released him about a year later, thinking he was a civilian agitator rather than a military threat.

It was not until July 4, 2014, that he seized the world's attention, climbing the pulpit of Mosul's medieval al-Nuri mosque in black clerical garb during Friday prayers to announce the restoration of the caliphate.

"God ordered us to fight his enemies," he said in a video of the occasion, which presented him as "Caliph Ibrahim, commander of the faithful".

Thousands of volunteers flocked into Iraq and Syria from around the world to become "Jund al-Khilafa" - soldiers of the caliphate and join him in his fight against the Shi'ite-led Iraqi government and its US and Western allies.

At the height of its power in 2016, Islamic State ruled over millions of people in territory running from northern Syria through towns and villages along the Tigris and Euphrates river valleys to the outskirts of Baghdad.

The group claimed responsibility for or inspired attacks in dozens of cities including Paris, Nice, Orlando, Manchester, London and Berlin, and in nearby Turkey, Iran, Saudi Arabia and Egypt.

In Iraq, it staged dozens of attacks on predominantly Shi'ite Muslim areas. A truck bomb in July 2016 killed more than 324 people in a crowded area of Baghdad, the deadliest attack since the 2003 US-led invasion of Iraq.

The group also carried out many bombings in north-east Syria, which has been under the control of US-backed Kurdish forces.

Most of al-Baghdadi's speeches were distributed as audio recordings, a medium better suited to the secretive, careful character that for a long time helped him evade the surveillance and air strikes that killed more than 40 of his top commanders.

That caution was matched by ruthlessness as he eliminated opponents and former allies, even within Salafi jihadist ranks. He waged war on al-Qaeda's Syrian wing, the Nusra Front, breaking with the movement's global leader, Ayman al- Zawahiri, in 2013.

By the time of his death, his fortunes – and those of IS – were in rapid decline.

With the defeat of IS in its stronghold Mosul, which he declared as the capital of his caliphate, in 2017 the movement lost all the territory it once controlled in Iraq.

In Syria, IS lost Raqqa, its second capital and centre of operations, and eventually earlier this year its final chunk of territory there when US-backed Kurdish-led forces took back Baghouz.

Was he a threat to other countries?

Al-Baghdadi repeatedly called on his followers to attack a list of enemies that came to include much of the world, including the United States and other Western countries, Shiite Muslims whom he deemed apostates, and even devout Sunni Muslims who rejected his group's ideology.

Unlike Osama bin Laden and other jihadists who strove to carry out 9/11-style attacks that would capture world attention, al-Baghdadi exhorted followers to do whatever they could with the weapons they had at hand. His group claimed scores of attacks worldwide, including so-called lone wolf attacks with no direct connection to the group.

But IS also directly orchestrated attacks, including the 2015 shootings and suicide bombings in Paris that killed 130 people. It also claimed this year's Easter suicide bombings in Sri Lanka that killed 269 people.

The extremist group attracted tens of thousands of foreigners to whom it provided advanced military training, and spawned powerful affiliates in Egypt, Libya, Afghanistan and elsewhere that continue to carry out attacks.

Most security experts believe Islamic State remains a threat through clandestine operations or attacks.

IS is believed to have sleeper cells around the world, and some fighters operate from the shadows in Syria's desert and Iraq's cities, still launching hit-and-run attacks.

In his most recent audio message, in September, al-Baghdadi put on a brave face, saying operations were taking place daily and urging followers to secure freedom for women jailed in Iraq and Syria over their alleged links to the group.

But the loss of territory in Iraq and Syria stripped him of the trappings of caliph and made him a fugitive in the desert border area between the two countries.

He was forced to travel incognito in ordinary cars or farm pick-up trucks between hideouts on both sides of the border, escorted only by his driver and two bodyguards.

What effect will his death have?

As the world's most-wanted terrorist with a $US25 million bounty on his head, al-Baghdadi's ability to run the day-to-day affairs of IS was probably very limited. He would have had to move among various safe houses with a small group of loyalists and avoid using electronic communications that could be tracked by intelligence agencies.

But he was an imposing figurehead, and his ability to elude the world's most powerful intelligence services for so many years added to his mystique among his followers. He proved to be a highly effective leader and will be hard to replace.

Al-Baghdadi never publicly designated a successor, and many of his top deputies have been killed. His death could spark infighting among prospective successors, potentially further weakening the group.

His two main Iraqi aides had been Iyad al-Obaidi, his defence minister, and Ayad al-Jumaili, his security chief. The two have been believed to be among the likely candidates for his succession but Jumaili was killed in April 2017 and Obaidi's whereabouts are unknown.

In any case, their military background and lack of religious credentials mean that any of al-Baghdadi's deputies would struggle to inherit his claims to be caliph.

Is this the end of Islamic State?

The Islamic State group in its various forms has survived the death of several leaders and senior commanders. It has been able to replenish its ranks by attracting Sunni Muslims in the Middle East who feel oppressed by their governments, as well as foreigners attracted by the group's austere vision of Islam, its ultra-violent tactics, or both.

It still boasts powerful affiliates in other countries, and remnants of the original group continue to carry out sporadic attacks in both Syria and Iraq.

Perhaps even more worrying are the tens of thousands of IS fighters and supporters detained across the Middle East, including those held by Kurdish fighters in eastern Syria. The US decision this month to pull out of Syria and abandon its former allies to a Turkish invasion allowed hundreds of IS supporters to escape and raised concerns about the security of other facilities.

It's possible that a future IS leader is wearing a prison jumpsuit, quietly recruiting supporters within concrete walls lined with barbed wire and plotting his next move – just as al-Baghdadi once did.

Reuters