In the final weeks of the 2016 election campaign, voters in swing states including Nevada and North Carolina saw ads appear in their Facebook feeds and on Google websites touting a pair of controversial faux-tourism videos, showing France and Germany overrun by Sharia law. French schoolchildren were being trained to fight for the caliphate, jihadi fighters were celebrated at the Arc de Triomphe, and the "Mona Lisa" was covered in a burka.