AP Photo

In the days of the Ottoman Empire, British diplomats referred to the Arabic-speaking territories of the empire as Turkish Arabia. It was these Arabic-speaking lands that Britain and France, in the aftermath of the First World War, divided into the modern Arab states we know today: Syria, Iraq, Jordan, and Lebanon. Those arbitrary colonial boundaries have endured for the better part of a century, but the people within them have never fully acknowledged the legitimacy of the lines that British and French officials drew for them.

Tribal confederations that span the borders, adjacent river towns, minority co-religionist communities—in these places, people have continued to live as they had done for centuries, intermarrying, trading, fighting, and migrating with...