Until recently, the conventional wisdom in Berlin held that a major terrorist attack attributed to one of the more than 1 million refugees and migrants allowed into Germany since mid-2015 would spell doom for Chancellor Angela Merkel, who faces a reelection bid in September.
Then, on Dec. 19, just such an attack occurred. A man plowed a stolen truck into a crowded outdoor Christmas market in Berlin, killing 12 people. Police sought a suspect who, they said, was a Tunisian refugee who had sought asylum in Germany. Islamic State claimed responsibility for the attack.
And then an odd thing happened.
Merkel’s popularity did not fall off a cliff. In fact, it has...