In four years, an unattractive anniversary will pass by—let us hope unnoticed—in Europe. It was in the year 1010 a.d. that a large number of French Jews were exiled or murdered. Many historians believe this event inaugurated hundreds of years of European anti-Semitism. Similarly, many observers thought the horror of Hitler's Holocaust of the Jews—the Shoah—would inoculate Europe for all time against one of Christendom's most malignant viruses: anti-Semitism. Sadly, they were wrong.

In the middle of the first decade of the third millennium, anti-Semitism in Europe has made a horrifying comeback. One of the most dramatic examples was the February murder in France of Ilan Halimi, a 23-year-old Jewish cell-phone salesman. This young man was kidnapped, ostensibly because it was thought he would fetch a good ransom (his kidnappers said they thought all Jews were "rich").

The chief rabbi of Great Britain, Sir Jonathan Sacks, has described a "tsunami-like" wave of anti-Semitism spreading across Europe and around the world. Many of his European rabbi-colleagues, he says, have been attacked in the street while wearing the traditional Jewish skullcap, the yarmulke. Jews have been singled out for insults, if not for assault, on streets of cities across the old continent. A British organization, the Community Security Trust, recently claimed that anti-Semitic incidents in Britain are at the highest level since it started keeping records in the 1980s. The World Jewish Congress goes further: Anti-Semitism in Europe, it says, is now worse than at any time since 1945.

Part of the explanation is a rising tide of both anti-American and anti-Israeli sentiment in mainstream European politics. Of course, it is entirely legitimate ...