TIMBUKTU, Mali—French tanks were closing in on this storied caravan city on the night of Jan. 23, when the al Qaeda-backed militants who had governed Timbuktu since April left a departing blow. They broke into one of the world's most valuable libraries, ripping centuries-old manuscripts from shelves.

Then they torched these priceless artifacts, in a scene of destruction that horrified scholars around the world.

But...