Who is al-Baghdadi?
He's an Islamic scholar, poet, and Sunni extremist who is as much as an enigma to his followers as he is to his enemies.
Born Awwad Ibrahim Ali al-Badri al-Samarrai in the central Iraqi city of Samarra, Abu Bakr al-Baghdadi, 43, is believed to have started his career as a preacher of Salafism, a hard-line form of Sunni Islam, and to have a degree in history and a doctorate in sharia law. After the U.S. invaded Iraq in 2003, he led a Sunni militant group that fought against American troops. Captured by U.S. forces in 2005, he was held for four years at Camp Bucca, a U.S. military prison. There, he met several al Qaeda commanders. In 2009, the U.S. turned al-Baghdadi over to Iraqi authorities as part of a Bush administration agreement with the Iraqis. Col. Ken King, who oversaw Camp Bucca, recalls al-Baghdadi taunting his American captors at the time, "I'll see you guys in New York." He was quickly released by the Iraqis and used his prison contacts to take over an al Qaeda–aligned militant group, the Islamic State of Iraq. Shortly after, he began an offensive to seize territory.

What is his goal?
Al-Baghdadi has the megalomaniacal aim of restoring the long-expired caliphate, the original Muslim kingdom that existed under the successors of the Prophet Mohammed and at one point extended from modern-day Spain to Central Asia. "Caliph," or khalifa, means "successor" in Arabic, and by taking the title, al-Baghdadi has declared himself the chief imam and political and military leader of all Muslims. The last caliphate ended with the collapse of the Ottoman Empire in World War I, and establishing a state to be the home of the faithful has been the dream of Islamic fundamentalists for more than half a century. Al-Baghdadi claims to trace his lineage to the Prophet Mohammed's Quraysh tribe, and his nom de guerre recalls the first caliph: Abu Bakr, father-in-law and close adviser of Mohammed. In July, he addressed the world's Muslims in a sermon. "I am the wali [leader] who presides over you," al-Baghdadi said at the Grand Mosque in Mosul, Iraq. "Obey me as long as I obey God in you."

What kind of state does he want?
At its height 1,000 years ago, the Islamic caliphate was the world's center of science and art, a beacon of tolerance during Europe's Dark Ages. Al-Baghdadi, by contrast, draws inspiration from the earliest form of the caliphate, when the first four successors of Mohammed spread Islam by sword in the 7th century. He also admires the Abbasids, the dynasty of caliphs who founded Baghdad in the 8th century. He models his justice system after theirs — using beheadings, stonings, and crucifixions. "Terrorism is to refuse humiliation, subjugation, and subordination [to infidels]," al-Baghdadi said in July. "Terrorism is for the Muslim to live as a Muslim."

How did he become so powerful?
By exploiting the Syrian civil war. When al-Baghdadi took over the Islamic State of Iraq, he came into conflict with al Qaeda's central leadership, which chastised it for fighting Shiites instead of Westerners. Al-Baghdadi defiantly sent his fighters into Syria to seize land and renamed his group the Islamic State of Iraq and Syria — a power grab that caused al Qaeda leader Ayman al-Zawahiri to disown him. By then, al-Baghdadi had tapped into al Qaeda's primary funding sources in the Persian Gulf, and when his army seized oil fields in Syria, ISIS became a juggernaut.

What does al-Baghdadi control?
ISIS is now an extraordinarily well-armed and well-funded militant group. Once al-Baghdadi's fighters entered Syria, he rallied other jihadists to his banner and captured large chunks of territory. At least 10,000 militants are now loyal to him, and more flock to his cause every week. The group is headquartered in the city of Raqqa, Syria, from which it governs a Massachusetts-size territory that straddles parts of Syria and Iraq. ISIS funds its expansion by selling oil, collecting taxes, looting banks, and selling antiquities; it is believed to control some $2 billion in cash and assets, and has amassed vast quantities of weaponry, including hundreds of millions of dollars' worth of U.S.-supplied artillery and armored vehicles captured from fleeing Iraqi forces.

What is life under ISIS like?
It is medieval. ISIS has imposed the most brutal form of sharia law throughout its territory, destroying Shiite mosques and tombs; slaughtering those it deems infidels; and banning music, drinking, and smoking. Women must wear a face veil, and all residents must attend prayers five times a day. Thieves have their hands amputated, while women suspected of adultery or immodesty are flogged. Thousands of minority Christians and Yazidis have been slaughtered. The U.S. has put a $10 million bounty on al-Baghdadi's head, and he's believed to be on the official "kill list" for drone attacks. "Now that he has claimed the caliphate," said Charlie Cooper, a British counterterrorism analyst, "he has effectively positioned himself as the standard-bearer of jihadism the world over."

Sowing terror via Twitter
ISIS is by far the most media-savvy militant group to emerge in the Middle East. Its social-media director is believed to be an American: Ahmad Abousamra, 33, a dual U.S.-Syrian citizen who was born in France and raised in the Boston area. Fluent in Arabic and English, he studied computer science at Northeastern University, where he made the dean's list. Under his influence, ISIS fighters are encouraged to use Twitter and Facebook to promote jihad, and many of them have posted photos and videos of themselves holding up severed heads or executing prisoners. "ISIS understands very well that in order for an act of terrorism to be effective, it needs to actually terrorize people," says Peter Neumann of the International Center for the Study of Radicalization. "The act of communication that follows the act of violence is almost as important as the act of violence itself."