In his much discussed article "Al-Qaeda's Fantasy Ideology" Lee Harris essentially argues that 9/11 was not an act of war because that would describe it in too rational a way. He dismisses militant Islamism as a fantasy without rational goals. He makes the mistake he criticizes others for - he makes an inaccurate judgement about an exotic culture. For millions of Muslims throughout 1500 years of history battling infidels and creating a caliphate are Allah's will and completely rational goals as far as they are concerned. In that context 9/11 was an act of war against the "house of war" the part of the world still not under Islam.
I do think his Hitler/Nazism analogy is weak; National Socialism, at least as it developed under Hitler, was a weird political aberration perhaps even a fantasy. However, even if we dismiss the Koran as one man's dreams, the fact is there are centuries of Muslim history, politics and culture and millions of people who believe it to be the literal word of God. Faith may sometimes be irrational, but pursuing a goal is often quite logical.