How did ISIS gain this momentum? One word: Success. By gaining ground against Assad, ISIS gained the respect of many young Muslims. How much more popular is ISIS going become now that they're standing their ground against the United States?
ISIS has recruitment centers across Iraq.
(CNN) — Abu Raad pleaded with his son not to volunteer.
An ISIS Recruitment Center in Mosul
But there was nothing he says he could say to talk his 19-year-old son out of joining ISIS, which refers to itself as the Islamic State and is formerly known as the Islamic State of Iraq and Syria.
"I don't know what they told him," Abu Raad told CNN by telephone from his home in Mosul. "Last week, my son came home and told me that he is leaving home and joining the Islamic State."
And then, the teenager packed a bag and left.
"We have heard nothing from him," said Abu Raad, who asked to be identified by his Arab nickname out of fear of retaliation by ISIS for speaking out.
His son is now believed to be one of hundreds of young Sunni men who, two Iraqi senior defense officials tell CNN, have joined ISIS in recent weeks in the Iraqi provinces of Nineveh, Salaheddin and Anbar.
Analysts and U.S. officials estimate ISIS has as many as 10,000 fighters in Iraq and Syria, including those who were freed from prisons by ISIS and Sunni loyalists who have joined the fight as the group advanced.
But that number likely doesn't include these latest recruits, mostly young men between the ages of 16 and 25 who are primarily poor, unemployed and lack an education, the two Iraqi senior defense officials told CNN.
Add to that a disenfranchisement felt by Iraq's Sunni minority, who have bitterly complained of being marginalized and cut out of the political process by Prime Minister Nuri al-Maliki's Shiite-dominated government.
The height of that disenfranchisement coincided with ISIS routing Iraqi security forces in Mosul, Iraq's second-largest city, in June.
Abu Raad blames al-Maliki's government for what happened with his son.
"He allowed those (ISIS) thugs" to take hold in the country, he said.
U.S. President Barack Obama vowed on Saturday to push Iraq's lawmakers to create a more inclusive government that "can give confidence to populations in the Sunni areas that (ISIS) is not the only game in town."
But for now, at least in Mosul, ISIS is recruiting young men at a rapid pace, officials and residents told CNN.
"Once they are in, (ISIS) gives them cars to drive, guns, cell phones and cash money," said one man, who lives in Mosul and has direct knowledge of ISIS recruit efforts.
In Mosul, for example, ISIS recruiting efforts appear to begin at information centers that have been set up around the city.
Pictures of the centers posted online purportedly by ISIS claim to show militants, many dressed head-to-toe in black, distributing leaflets, videos and CDs about their operations to men and boys.
While CNN cannot confirm the authenticity of the pictures, they appear to coincide with descriptions provided by the man who described ISIS recruiting efforts.
The pictures also show men and young boys gathered in front of the centers, watching videos purporting to show ISIS leader, Abu Bakr al-Baghdadi, on giant flat-screen televisions.
In some of the videos, al-Baghdadi is heavily armed. In others, he appears to be preaching at a pulpit.
Still other pictures appear to show videos about bomb-making and suicide bombs.
At the centers, ISIS encourages young men to join, according to a man who asked to be identified only as Abu Younis out of fear of retaliation by ISIS
The actual recruiting occurs elsewhere, according to Younis, who lives in Mosul and told CNN he has visited the centers.
Sometimes, he says, recruiting takes place at mosques. (Continue Reading.)