Victor Davis Hanson is writing on historical islamic imperialism. It is well worth the read. Here is an excerpt:

But unlike the West, where Christianity began as the persecuted victim of the Roman Empire and conceived of its own empire as a spiritual one apart from the political realm, Islam “was inextricably lined with the creation of a world empire and its universalism was inherently imperialist. It did not distinguish between temporal and religious powers.” Muhammad could thus “cloak his political ambitions with a religious aura,” a rationalization of domination that drives the contemporary jihadists like Osama bin-Laden and Ayatollah Khomeini, who just like centuries of warriors before them justify their aggression by quoting Muhammad’s farewell address, in which the Prophet laid down the injunction “to fight all men until they say ‘There is no god but Allah.’”