Muslims are fond of saying that the terrorists are only reacting to the imperialism of the West, that our cultural and military encroachments are in every way to blame for their terrorism.  However, this is untrue.  The real reason is verse 9:29 of the Koran:

[9.29] Fight those who do not believe in Allah, nor in the latter day, nor do they prohibit what Allah and His Apostle have prohibited, nor follow the religion of truth, out of those who have been given the Book, until they pay the tax in acknowledgment of superiority and they are in a state of subjection.

Islamic theology teaches that the later verses in the Koran abrogate the earlier verses.  Chapter 9 is one of the latest chapters, and possibly the latest.  Therefore, this verse advocating warfare against non-Muslims is one of the most authoritative in the entire Koran, and it applies to all unbelievers in all circumstances.  Don't let a Muslim tell you otherwise!  Muslim scholars almost universally recognize this verse as applying today and in every situation relating to Muslim interactions with non-Muslims. 

This is yet another prime example of how inconsistent the Koran is with the Bible.  Jesus specifically commands us to 'turn the other cheek,' :

Matt 5:39 But I say to you, Do not resist the one who is evil. But if anyone slaps you on the right cheek, turn to him the other also.

It should be obvious that this verse directly contradicts verse 9:29 of the Koran.   What is the usual Muslim response when presented with Jesus' words?  That Christians changed the Bible so that it no longer agrees with the Koran, even in it's earliest form.  The Muslims have a major hurdle to overcome from their own book to defend this view, however:

5: 46-47 And in their footsteps We sent Jesus the son of Mary confirming the Law that had come before him: We sent him the Gospel: therein was guidance and light and confirmation of the law that had come before him: a guidance and an admonition to those who fear Allah. Let the people of the Gospel Judge by what Allah hath revealed therein. If any do fail to judge by (the light of) what Allah hath revealed they are (no better than) those who rebel.

You see, the Koran itself claims that the Bible is true.  It's not that Christians changed the Bible, it's that Muhammad was ignorant of the Bible and its contents.  If Muslims claim that we changed the Bible to disagree with the Koran, what about verse 5:46-47?  Usually, they'll dodge this question.