It's a pretty complex issue and I'm neither an expert or a muslim but I'll try to weigh in.

One thing people have to keep in mind is that it is a bit wrong to equate Christianity with Islam culturally, though they share many fundamental similarities they also have very important differences.

During the height of Christianity we loosely had "Christendom" similar to how Muslims (to my understanding) view the Muslim world today. It is a religious affiliation above a national or even ethnic one.

Now some muslims are angry at the west for several reasons. They have a cultural memory that lasts longer then the west's tends to, so they still see the crusades as having solidified the west or Christianity against Islam.

So what they perceive as slights against Islam they attribute to the west in general. These insults are also double edged, damn the west for be complaisant with our dictators or fuck the west for intervening in our matters too much.

Since they have the sense of a Muslim world then it is easy for a radical in Algeria to feel the need to jihad for muslim's in Iraq, because they are a Muslim before a national. This increases the potential population that could be radicalized drastically.

Imagine if during the troubles in Ireland Catholics in Africa, Italy, South America decided to commit terrorists acts because protestants were attacking Catholics, regardless of whether or not the reason was nationalist and completely divorced from effecting their world. (Which is why it is a bit false to compare Christian terrorists with Islamic terrorists.)

To add to all of this it isn't every muslim that believes this. Recently, as in the past 100 years, a school of Islam started by the Saudi royal family called "wahhabi" believes any modernization is anti-Islamic and attempt by the west to take over the Islamic world.

They wish to destroy any modernization, they see it as taking the world away from Islam (i.e. the Taliban banning movies) and they see the west as the main perpetrators.

To make things worse, since wahhabism was started in Saudi Arabia, it is a school of thought with a lot of money behind it. For the most part if a family wants there kid to have some Islamic schooling their only option is a wahhabi funded school, or if they want any schooling at all it is often only wahhabi funded schools.

So since the war on terror many uninformed and uneducated muslims have been taught by Imams that their shitty lot in life is entirely the fault of the west, the war on terror being a great way for the to say "see! the west is massacring your muslim brothers and sisters!".

And the only way to get Islam to its former glory during the caliphates is to jihad in the way they are doing it.

I could be entirely wrong about a lot this so someone please correct me.