called ISIS for short, but it's easier to understand what they are if you know them

by their old name, al-Qaeda in Iraq. They were a key part of the insurgency after America's

2003 invasion toppled Saddam Hussein. And at the time, they had a lot of support from

the country's minority Sunnis. They held a lot of territory because Sunnis were furious,

both at the Americans who had kicked Saddam Hussein out of power, and the Shias who had

taken over the government. But al-Qaeda in Iraq had big ambitions. They didn't just want

to kick America out. They wanted to set up an Islamic state. So they banned music, and

they banned smoking. Women couldn't show their hair and they began beheading civilians who

disobeyed their rules. Starting in 2006, their brutality lost them the support of Iraqi Sunnis,

who partnered with US forces to help push al-Qaeda in Iraq out of the country. America

takes a lot of credit for this. We call it the surge. We're very proud of it and we should

be, but the big thing we did in the surge was we helped the Sunnis, Sons of Iraq, as

they called themselves, rise up against al-Qaeda in Iraq's savage theocratic rule. So al-Qaeda

in Iraq was defeated, but they weren't destroyed. They were driven out of much of the territory

they used to control, and then they began rebuilding. In particular, they became heavily

involved in the fighting in Syria. They were trying to overthrow the Shiite Assad regime.

Their tactics in Syria were so brutal and their ambitions were so grand that al-Qaeda

itself actually cut ties with them. Al-Qaeda thought al-Qaeda in Iraq was bad for the al-Qaeda

brand. After their split with al-Qaeda in February 2014, they renamed themselves ISIS

and they began setting up extortion rackets from Syria. They took over part of the oil

industry. They sold electricity back to the Syrian government that they were fighting. Meanwhile

in Iraq, Prime Minister Malaki had been ruling on sectarian lines. Maliki is a Shiite and

he was empowering Shias. He was violently breaking up Sunni protests. He was arresting

Sunni politicians. So the minority Sunni population began to hate him and fear him. So ISIS returned

to Iraq and they began selling themselves as the Sunni champion against Maliki. And

they gradually grew strong -- strong enough to challenge the Iraqi government in the country's

second-largest city, Mosul. So when 800 ISIS soldiers challenged 30,000 Iraqi army troops

in the mostly-Sunni city, the Iraqi troops, most of them, they put down their arms and

ran. This was a group that was largely Sunni. They simply refused to fight and die for a

Shiite government that they didn't think cared for them. That ultimately is where ISIS's

real power comes from. They get weapons and they get money from the territory they control

and they have skilled fighters, but they're facing a government that's widely unpopular

among Sunnis. Only a tiny percentage of Sunnis actively support ISIS's goals, but at least

for now they appear willing to let ISIS operate in their territory freely because they see

it as an alternative to a Shiite government they despise. But ISIS wants do to much more

than that. They want to set up a new state that reaches up into both Iraq and Syria,

one that governs according to a medieval interpretation of Islamic law. And so Iraq's Sunni population

is caught between two terrible forces: A Shia majority that violently represses them to

hold on to political power, and a theocratic militia that will kill them

if they step too far into modernity.