SWC Poll: What Motivates Islamist Terrorism against the West?

[tequila]

I'd like to get a poll of what SWC Members think, especially in the wake of thought-provoking post by David Kilcullen.

What motivates Islamist terrorism against the West?

I'm not speaking about Islamist terrorism against, say, Shia marketplaces or Algerian parliament buildings, but rather against Western targets: the Twin Towers, the Pentagon, Madrid rail stations, and London subways.

1) Is it principally the foreign policy of the West? No ideological judgment about whether said policy is correct or not required here.

2) Or is it principally an inevitable cultural/religious/ideological clash?

3) Something else?