ISIS insurgents fired mortars at the oil refinery in Baiji, forcing the temporary shutdown of Iraq's largest gasoline processing plant. Here's what you need to know about the strategic importance of this refinery. (Davin Coburn and Kate M. Tobey/The Washington Post)

On the eve of the U.S. invasion of Iraq in 2003, a 36-year-old Jordanian who called himself “the Stranger” slipped into the suburbs of Baghdad armed with a few weapons, bags of cash and an audacious plan for starting a war he hoped would unite Sunni Muslims across the Middle East.

The tattooed ex-convict and high school dropout had few followers and scant ties to the local population. Yet, the Stranger — soon to be known widely as Abu Musab al-Zarqawi — quickly rallied thousands of Iraqis and foreign fighters to his cause. He launched spectacular suicide bombings and gruesome executions targeting Americans, Shiites and others he saw as obstacles to his vision for a Sunni caliphate stretching from Syria to the Persian Gulf.

Zarqawi was killed in a U.S. airstrike in 2006, but the organization he founded is again on the march. In just a week, his group — formerly known as al-Qaeda in Iraq and now called the Islamic State of Iraq and Syria, or ISIS — has seized cities and towns across western and northern Iraq at a pace that might have astonished Zarqawi himself. Already in control of large swaths of eastern Syria, the group’s black-clad warriors appear to have taken a leap toward realizing Zarqawi’s dream of an extremist Sunni enclave across the region.

It is unclear whether ISIS’s gains will last or whether the Sunni tribesmen who apparently aided the jihadists will submit to living under the group’s harsh brand of Islamic law. Either way, U.S. and Middle East officials say the group’s achievements are both remarkable and alarming, displaying the same mix of audacity, cunning and political skill that made Zarqawi such a fearsome opponent a decade ago.

Counterterrorism officials who tried to defeat the group during the Zarqawi era expressed begrudging respect for ISIS’s ability to recover from virtual extinction in the years after his death. The current leader, a former Iraqi teacher known as Abu Bakr al-Baghdadi, managed to find new purpose in the Syrian conflict and renewed strength in the lawless regions of eastern Syria and western Iraq, where his fighters could train and plan without interference from U.S. and other Western military forces.

How the Islamic State is carving out a new country

“They get sick, but they never die,” said a senior Middle Eastern intelligence official who has closely tracked the fast-moving developments in Iraq.

The official, who insisted that his name and nationality not be revealed in discussing his country’s intelligence assessments, said ISIS’s astonishing recent gains were mostly due to skillfully forged alliances with Sunni tribal leaders, with the group exploiting widespread resentment toward the Iraqi government in Baghdad.

“They all share the same hatred for the ruling regime,” said the official, citing examples of Iraqi Sunni collusion in ISIS’s sweep through Mosul, Tikrit and other Iraqi cities. “It would not be possible for a group like [ISIS] to control so much territory on its own. They are getting a lot of help from the sons of the tribes.”

So far, at least, ISIS has managed to avoid alienating its natural Sunni allies the way Zarqawi famously did. The Jordanian’s indiscriminate and unflinchingly brutal attacks on Iraqi civilians helped give rise to the Sunni backlash known as the Anbar Awakening, in which tribal sheiks withdrew their support from Zarqawi and actively helped U.S. forces find and destroy his operatives.

Even veterans of the Awakening seem more willing to give ISIS a chance. Zaydan Aljabri, an Iraqi tribal sheik from Anbar province, said he thinks ISIS has learned from the mistakes of the Zarqawi era and is now truly committed to defending Sunnis against a Baghdad government he sees as corrupt and repressive.

“If al-Baghdadi asked for my allegiance, I would give it to him, because what he is doing is what I want,” Aljabri said in an interview. “Right now, my priority is liberating the Sunnis, and that’s why he has been fighting for these last six months.”

Aljabri said Anbar’s Sunnis believe they can manage ISIS and prevent it from imposing the kind of harsh Islamic law that tarnished the group’s image in parts of Syria it controls.

“We are an Islamic country, but we want to be a developed country, to be part of the world,” he said. “Al-Baghdadi will not dare try to impose [sharia] law in Iraq, because he knows the tribes will not tolerate it.”

Conflicting signals in Syria

ISIS’s record in Syria offers conflicting signals on how it would seek to administer newly captured cities in Iraq. Baghdadi has repeatedly insisted that his group has learned from its past errors, and his lieutenants in Syria have tried to portray themselves as committed to improving social welfare. The group’s propaganda wing frequently posts videos of ISIS soldiers passing out food and blankets or clearing trash from Syrian streets.

But ISIS’s social-media sites are also filled with graphic images of Islamists carrying out public executions and amputations on suspected lawbreakers and ­beheading and mutilating pro-govern­ment fighters — and even members of rival militia groups. Already, in Mosul, ISIS has issued a new charter spelling out the creation of an Islamic state, along with strict new laws.

ISIS, meanwhile, has continued its campaign of attacks on Iraqi Shiite civilians, routinely bombing Iraqi markets, bus stops and schools.

The group’s embrace of extreme violence has drawn condemnation from al-Qaeda. Ayman al-Zawahiri, who assumed command of al-Qaeda after Osama bin Laden’s death, last year ordered ISIS to disband and leave Syria, and his organization issued a statement in February publicly disavowing ISIS. Zawahiri has embraced a rival Islamist rebel group, Jabhat al-Nusra, created in 2012 by a former deputy of Baghdadi’s.

Strikingly, the same divisions over tactics and ideology have been present since the earliest days. Zarqawi used graphic violence as a calling card, embracing brutal tactics that, while repulsive to most people, also made him an icon and hero — a fearless warrior who exacted painful revenge for the decades of humiliations and defeats borne by Muslims.

“Zarqawi loved the limelight, and his was an easy face to hate,” said former CIA officer Bruce Riedel, a counterterrorism expert who has written extensively about the Jordanian and his legacy. “But the fact was that an easy face for us to hate made him attractive to so many other people.”

Born as Ahmad Fadeel al-Khalayleh in the gritty Jordanian city of Zarqa, the movement’s founder gained a reputation as a brawler and a thug before discovering Islam in his early 20s. When he was 24, he left Jordan in what became the first of two life-changing events: a stint fighting communist forces in Afghanistan, followed by five years of incarceration in Jordan for his role in a foiled terrorist plot by local jihadis. He emerged from prison in 1999 as a hardened and committed Islamist with a cadre of disciples eager to follow him.

He returned to Afghanistan after his release from prison, but, distrusted by bin Laden, he established his own training camp in the western Afghan city of Herat, hundreds of miles from bin Laden’s base at Kandahar. In late 2001, when U.S. warplanes began bombing Taliban strongholds throughout Afghanistan, he sought refuge in the rugged mountains of northeastern Iraq, in a Kurdish region regarded as beyond the reach of Iraqi leader Saddam Hussein. There, he briefly joined forces with Kurdish militants and began a series of crude, unsuccessful attempts to create weapons from common poisons such as cyanide.

Zarqawi’s Iraqi camp eventually attracted the attention of the George W. Bush administration, which made Zarqawi a central pillar of its effort to link Hussein to al-Qaeda. Secretary of State Colin L. Powell mentioned Zarqawi 20 times in his Feb. 5, 2003, speech before the United Nations making the case for war against Iraq. Famous almost overnight, Zarqawi began receiving cash and support from sympathizers from Europe to Central Asia.

The administration’s allegations of operational ties between Zarqawi and Hussein’s forces proved to be false. But Zarqawi did use his Iraqi sanctuary to prepare for what he believed would be the ultimate confrontation between the Islamists and the United States. Within weeks of the U.S. invasion, he had established connections with Iraqi opponents of the occupation, as well as a Syrian network for delivering cash, weapons and volunteers into western Iraq.

Six months after the U.S. invasion, he launched a series of spectacular car bombings that became his signature, targeting in quick succession an embassy, a Shiite mosque and the local headquarters of the United Nations. Scores were killed, including Sergio Vieira de Mello, the popular head of the U.N. mission to Iraq.

“It was a brilliant strategy,” said Riedel, the former CIA analyst. “By attacking the U.N., he drove out all the nongovernmental organizations and discouraged anyone from opening an embassy. Then he went after the ­Shia-Sunni fault line with attacks on the Shiite mosques. So first he isolated us in Iraq, then he put us in the midst of a civil war.”

The Jordanian also would seek to strike fear into Americans and other Westerners in Iraq with a series of kidnappings and videotaped beheadings. The first victim, Pennsylvania businessman Nicholas Berg, was butchered on camera by a hooded Islamist that CIA officers later confirmed was Zarqawi himself.

His soaring profile eventually compelled al-Qaeda to formally induct him into its network. Bin Laden personally declared Zarqawi to be the “emir of ­al-Qaeda in the Country of the Two Rivers” and ordered jihadists in Iraq to obey him. Still, senior al-Qaeda leaders would privately scold the Jordanian over his indiscriminate killings of civilians.

“Let us not merely be people of killing, slaughter, blood, cursing, insult and harshness,” one of bin Laden’s deputies wrote in a letter to Zarqawi in late 2005.

But the Jordanian shrugged off the warnings, predicting that his signature attacks against Shiite civilians were necessary to prod Sunni Muslims into joining a war for their own liberation. “If we succeed in dragging them into the arena of sectarian war, it will become possible to awaken the inattentive Sunnis, as they feel imminent danger,” Zarqawi wrote to bin Laden in a 2004 letter intercepted by Western intelligence agencies.

After Zarqawi

Zarqawi was killed on June 7, 2006, after an intensive intelligence operation tracked him to a safe house north of Baghdad where he was meeting with his spiritual adviser. A U.S. warplane dropped a pair of guided bombs on the building, destroying it. Zarqawi was pulled alive from the rubble but died minutes later.

His death prompted a re-branding of the group — which became known as the Islamic State of Iraq — but it slowly weakened under relentless pressure from U.S. intelligence and Special Operations forces and the rejectionist Awakening movement. But Baghdadi, who emerged as the group’s leader in 2010, saw in the Syrian civil war as an opportunity to reclaim legitimacy among Sunni Muslims. The group was renamed yet again, as the Islamic State of Iraq and Syria, and it soon surpassed other Islamist rebel militias in seizing territory and drawing foreign recruits.

ISIS moved aggressively into Iraq early this year by seizing control of Fallujah and other towns in the Sunni heartland. Factoring in the additional gains in the past week, the group now controls a swath of land stretching from northern Syria to central Iraq. Some who study Islamist militants see ISIS as on the cusp of an achievement sought by Zarqawi a decade ago: erasing the Western-imposed boundary lines that divide the Middle East into nation-states.

Zarqawi repeatedly boasted that he would undo the ­Sykes-Picot accord, signed secretly by Britain and France in 1916 as a basis for carving up the region, said Hassan Abu Hanieh, a Jordanian author and researcher of Islamist movements who knew Zarqawi during his years in Jordan.

“These [captured] territories already amount to an Islamic state,” Hanieh said. “In effect, they have canceled Sykes-Picot. The jihadists are there now, whether we like it or not.”

Warrick reported from Tel Aviv and Amman, Jordan.