As Arab thoroughfares go, Hamra Street in the center of Beirut is probably the most chic of them all. International in flavor, cosmopolitan in character, it boasts the sort of smart little café where a Lebanese sophisticate can pause between water-skiing in the Mediterranean in the morning and snow-skiing in the mountains just above the city in the afternoon. “The Paris of the Middle East” used to be the cliché about Beirut: by that exacting standard, I suppose, Hamra Street would be the Boulevard Saint-Germain.

Not at all the sort of place you would expect to find a spinning red swastika on prominent display. Yet, as I strolled in company along Hamra on a sunny Valentine’s Day last February, in...