Three audacious terrorist attacks on Dec 19 left the world horrified. Perhaps the most chilling of those attacks was the assassination of Russia's ambassador to Turkey Andrei Karlov in Ankara. The assassin, an off-duty police officer, pulled out a gun while Karlov was several minutes into a speech at an art exhibition and shot up to eight times, shouting "Don't forget Aleppo, Don't forget Syria".

In Berlin, a truck plowed through a Christmas market, killing 12 people, which German Chancellor Angela Merkel described as a "terrorist" attack. The suspect, a Tunisian national, was killed in a shootout with police officers in Milan, Italy, on Friday.

And in Zurich, Switzerland, three people were wounded when a gunman opened fire in a mosque frequented by Somali immigrants.