What a crisis Europe is in, with waves of migrants reaching its shores as the Arab world implodes. It is the biggest migration into Europe since the end of World War II and is shaping up to be its first great and sustained challenge of the 21st century. It may in fact shape that continent’s nature and history as surely as did World War I.

It is a humanitarian crisis. As Richard Haass of the Council on Foreign Relations notes, it will not soon go away, for two reasons. First, the Mideast will not be peaceful anytime soon and...