BERLIN—A Star of David crudely daubed on the doors of Jewish homes in Berlin. An Orthodox Jewish man punched in the face on a London bus. Threatening letters sent to a prominent Jewish politician in France. Across Europe, where centuries of pogroms and the Holocaust nearly wiped out Jews, those who remain have taken a double blow. The first is the grief and shock from the Hamas attack on Israel that shattered an assumption that at least there, Jews were safe from the kinds of attacks that mark their history in Europe.