This year the leaders of France and Britain declared that their countries' policies of multiculturalism had failed. As when Germany's Angela Merkel made similar statements last year, Nicolas Sarkozy and David Cameron sparked a political firestorm.

Europe's debate over multiculturalism, and how to deal with non-European immigrants, will only intensify as the full effects of the Arab Spring play out on our continent. But it's worth stepping back to consider how we arrived at this point—how it became so controversial for a Western...