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-Qaida 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’s unclear whether the gains of ISIS 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 the ability of ISIS 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 Western military forces.

“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 the astonishing recent gains of ISIS were mostly due to skillfully forged alliances with Sunni tribal leaders, as the group exploited 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 the sweep of ISIS 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.”