British citizens are doing the same. One is suspected in the murder of an American journalist.
CBS News looked at how Britain became a center for terrorist recruitment.
At home in Britain, he was Nasser Muthanna, a gifted 20-year-old who once dreamed of becoming a doctor. But in Syria, he is now Abu Muthanna al-Yemeni - one of hundreds of young men who have left the U.K. to join the brutal terrorist group ISIS.
The execution of American journalist James Foley, apparently at the hands of a British citizen, has left Britain struggling to understand how one of its own could be responsible for such an atrocity.
Shiraz Maher is an analyst at the International Center for the Study of Radicalization.
"It was a shocking thing to see but it wasn't surprising. If you look at British fighters in Syria over the last year, we've seen them operate as suicide bombers. We've seen them execute prisoners of war. They are participating in most of the brutal acts out there," Maher said.
He said these men no longer see themselves as being British.
"They've disengaged from British society, they've disengaged from British values, they've bought into something else -- a different identity," Maher said.
They see themselves as defenders of Islam in a war against nonbelievers. Their barbaric tactics have been glamorized on social media, a powerful recruitment tool that British authorities are now trying to crack down on.
But the challenge is a formidable one. With every victory on the battlefield, ISIS' appeal grows. Many are concerned that it is only a matter of time before their jihad spreads beyond the Middle East.
On a video posted on YouTube, ISIS foreign fighters are seen burning their passport.
"This is a message for to Canada and all the American power-elite, we are coming and we will destroy you," one jihadi says on the video.
"We are coming for you, Barack Obama!" he adds.
One thing that is striking about these jihadis is that some of them are religious novices. Two fighters who came from Britain reportedly purchased copies of "Islam for Dummies" on Amazon. So they don't necessarily have a sophisticated understanding of Islamic theology. They are drawn to the idea of being warriors.