What does radicalisation look like?

It is estimated that over 30,000 foreign fighters from 100 countries have entered ongoing conflicts in Syria and Iraq since 2011. These foreign fighters have travelled to the Middle East to join the Islamic State (often abbreviated as IS, ISIS, ISIL, or Daesh), a Salafist jihadist group that follows an ultraconservative branch of Sunni Islam. But what has driven these individuals to migrate to ISIS territory and serve the self-proclaimed Caliphate? Why have so many foreign fighters joined the ranks of Jihad? The underlying assumption is that the majority of European mujahideen have undergone a process of radicalisation and now believe that an offensive holy war is required to defend and expand Islam.