The ostensible purpose of the recent videos that show the beheadings of two American journalists by Islamic militants is to deter attacks — your missiles on our positions will beget our knives on Western hostages — but the true aim is to spread dread and terror.

What they made are modern media artifacts being used to medieval ends. The videos serve as both propaganda and time machine, attempting to wipe away centuries of civilization and suggest that the dreamed-of caliphate flourishes and blood is cheap currency.

We don’t want to look, but some of us do and the rest of us talk about it. ISIS seems to understand that the same forces that carried the Ice Bucket Challenge’s message of uplift — the desire to be part of something, to be in the know — can be used to spread fear and terror as well.