You could have asked the same question to the Roman Empire. It's decadence. Confronted with a threat, decadent cultures tend to navel-gaze, and to assume that the other side is just like them. The misapprehension that the other side wants to be left alone leades to a tendency to ignore, rather than to deal with the threat.
Steyn, BTW, nails the threat: it's not Islam per se, but rather the ideology and practitioners of Wahabism. Deal with those nice people, and the rest (and majority) of Islam will eventually emerge from the 7th Century.
Victor David Hansen was a guest on Savage the other night (Savage has been drawing better guests lately), and he made the argument that it isn't even about Wahabism. It's about the same culture that in the past used Pan-Arabism, Baathism, and Naziism to try to gain back the dominance of the glory days of the Ottoman Empire. Actually, he didn't mention the Ottoman Empire, that just occured to me. But his point was that they will ride whatever wave is available to help them acheive their goal, which is the destruction of Western Civ.
He said the point was well illustrated by Saddam who, starting as a communist, was building mosques by the end.
As I just said above, it is the Communists. The Roman empire was pagan and persecuted the Christians. With no abiding moral principles which stand independently of the government, a society will crumble on its own.
Who is fighting overtime to rid our country of religion, especially Christianity? No prayer in schools, no Ten Commandments, abortion, promotion of homosexuality, moral relativism, etc. It is the Democrat Party, performing as a stand in for the Communist Party, and we should stop skirting the issue. The Democrats are our enemies and must be treated as such.