Calling ISIS a "greater threat than we've seen before," Prime Minister David Cameron raised Britain’s terror level on Friday from “substantial” to “severe.” "The root cause of this threat,” he said, “[is] a poisonous ideology of Islamist extremism that is condemned by all faiths and faith leaders. It believes in using the most brutal form of terrorism to force people to accept a warped world view and to live in an almost medieval state.”

The march of ISIS across Mesopotamia, with its calculated public viciousness and its appalling human suffering, is unbearable to watch.

And yet, as others point out, ISIS is not just a terrorist organization. It has acquired territory, governs an (unrecognized) state and seeks to impose its ideology on the world. There is a word for this – imperialism – which is neither new nor Islamic. It’s what the Romans and – a millennium later – the Roman church did. It was the aim of both Stalin’s Soviet policy and Putin’s Russian expansionism. It propelled Hitler’s “Lebensraum” and Japanese militarism. Lenin called it the Highest Stage of Capitalism, and it was embedded in our own “manifest destiny,” as those in its way well understood. And who should know more about imperialism than Great Britain, which established, with staggering cruelty, an empire on which the sun never set?

It drives the ruthless, in the name of some greater go(o)d, while the rest of us hope only to live our short time here in peace.