LONDON • One would have thought that, after the unspeakable murders of the Holocaust in which six million Jews perished, anti-Semitism as an ideology justifying hostility and discrimination against Jews was discredited for ever.

But you would be wrong. For the resurgence of anti-Semitism is one of the key political challenges facing Europe today. In Britain, eight Members of Parliament resigned last month from Labour, the country's main opposition party, after they and their colleagues were subjected to repeated anti-Semitic taunts.