When a lone gunman assassinated Russia's ambassador to Turkey in a shocking public display at an Ankara photo exhibit yesterday, the international consequences were immediate. Hours later a tractor trailer truck mowed down holiday revelers celebrating at a Christmas market in Berlin, killing 12 and wounding scores more. Germany has been preparing for terror attacks like the ones that have struck in Paris and Belgium. While authorities are treating these isolated incidents as the work of terrorists, motives and details are still very unclear. One thing is certain: the perception of escalating violence could have far-reaching effects on Europe's policy on immigration, refugees and the war in Syria.