ERRIADH, TUNISIA // In 586BC the Babylonian king Nebuchadnezzar laid waste to Jerusalem, inadvertently sowing seeds for a Jewish haven across the sea that has outlived his realm by 25 centuries and counting. Legend tells that refugees fled to the Tunisian island of Djerba, carrying a block from the ruined Temple of Solomon. Today it lies beneath the El Ghriba synagogue, the cornerstone of a thriving Jewish community. And after decades of Jewish exodus from Arab countries, that community is growing. For western holidaymakers, Djerba is a strip of lavish resorts along a sandy Mediterranean coast. For Tunisians, it also...