How ISIS Became One Of The World's Most Dangerous Terrorist Groups

When the US withdrew its last troops from Iraq in 2011, Al Qaeda in Iraq had been defeated and one of the Middle East's most troubled countries had the chance to carve out a normal existence for itself.

Three years later it had all fallen apart.

Al Qaeda in Iraq turned into ISIS, seized Iraq's second-largest city, took over large sections of Iraq and neighboring Syria, and triggered a full-on regional crisis.

Between the divisively sectarian policies of former Iraqi Prime Minister Nuri al-Maliki and the chaos in neighboring Syria, al-Qaeda in Iraq quickly went from a broken organization operating at the fringes of society to a major military threat.

To make sense of the meteoric rise of ISIS, Frontline, PBS's award-winning news documentary series, produced an in-depth look at the group's ascendancy. Here's what they found about the rise of a jihadist group that has become one of the most consequential players in the Middle East.