“The biggest reason for that is that ISIS philosophically has welcomed all Muslims as equals, as it’s building an islamic state which does not have particular Syrian angle,” Landis said. “Also, ISIS’ leadership is made of people with very prominent roles that are foreigners so you’re not going to be discriminated against philosophically if you’re foreign.”

Social media also plays a significant role.

While in the past jihadist groups operated in secretive online forums, ISIS spreads its message—both in English and Arabic—on Twitter and Facebook, which are inherently open to the public. With its sleekly produced propaganda videos, ISIS reaches young, restless Muslims or other devotees around the world with a cause that they see is worth fight for, experts say.