One to one cause and effect rarely applies in the real world. Events have multiple causes, and that includes the event of adopting a fanatical ideology.

Sure it's a fallacy to identify one particular factor, such as the Iraq War, and say that's the sole cause of bombs in London, and ignore a whole set of other ideological factors. But it's equally fallacious to do the same for other factors, such as those young Hassan focused on in that article Wolfgang linked to there, and dismiss the importance of, for example the war.

What moves people into crazy action tends to be a combination of facts on the ground and ideology. That applies to left-wing terrorism, it applied to the IRA, it applies to the jihadis.