For Clarissa Ward's full reports, watch the CBS Evening News tonight and Wednesday at 6:30 p.m. ET.
After months of planning, I found myself in the back of a car speeding along the winding roads of Syria, near the border with Turkey.
I've been in and out of Syria about a dozen times since the civil war began more than three years ago. I looked out the window at the familiar silvery green olive trees dotted across ever-expanding camps for the millions of internally displaced people. Riding shotgun was my host, a 26-year-old former soldier from Holland who joined the jihad in Syria two years ago. He goes by his last name, Yilmaz. He gestured at the sprawling camp.
"Look at this, please. It's getting bigger and bigger every day. No education, no proper food... nothing for these people," he said. "They live with the bare minimum. They've been begging for help for so long. And what do they get, they get bombs. And then they ask me, would you fight these people that are bombing them. I would fight anybody. Even if it was my own father that was bombing them, I would fight him and kill him myself."
Yilmaz is one of the many Europeans to have joined the dozens of rebel groups -- including the Islamic State of Iraq and Syria (ISIS) -- across Syria and Iraq. Intelligence estimates put the number of Westerners who have joined the militants as high as 3,000. It's a worrying trend for Western governments, and sparked our interest in trying to find out why so many would give up their lives and join a war thousands of miles away. After four months of negotiations, Yilmaz had finally agreed to let me interview him. I traveled undercover across the bustling Turkish-Syrian border to meet him.
He said he decided to join the jihad to help defeat Syrian dictator Bashar Assad. He'd been deeply disturbed by the hundreds of YouTube videos posted every day showing regime's brutal crackdown in Syria. This is a war playing out in real time on the Internet. Yilmaz's experience in the Dutch military immediately made him a sought-after trainer for the rebels as more recruits of varying skills joined the cause. Over two years, Yilmaz's religious outlook became more fundamentalist. Although he doesn't fight with ISIS, he won't condemn them either.
Yilmaz introduced me to another foreign fighter, an American who goes by the nom du guerre Ibn Zubayr. Ibn Zubayr told me he left his home in the Midwest three years ago to come to Syria. He fights with Jabhat al Nusra, a rebel group that has sworn allegiance to al Qaeda and which has been designated a terrorist organization by the U.S. government.
Zubayr was an ordinary teenager in the U.S., playing high school sports, growing up on Hollywood movies and attending college. When he decided to study overseas in the Middle East, everything changed. He became politicized by the war in Syria, and decided he couldn't return home. Instead, he joined the jihad and has become radicalized by what he sees as a war on Islam, led by the United States.
His views are extreme: he has praise for Osama bin Laden and al Qaeda. The U.S. airstrikes have hardened his resolve -- he narrowly escaped death when a Tomahawk missile hit the house where he had been living in Idlib, in western Syria.
The sharp spike in the number of Western jihadis has triggered real concern among leaders in the U.S. and Europe. When they return home, will they launch terror attacks? Can they be stopped from traveling to the Middle East? What motivates them to do it, and how can governments prevent it?
Outside of the hundreds of online propaganda videos, we hear so little from these men and women, who trade in their comfortable lives in the West for an unfamiliar battlefield. Our conversations with two foreign fighters provide a rare insight into a war that's changing the face of the Middle East, and beyond.