Many Muslims look toward the long history of Islam to find clues about its future. For most of the history of Islam, Muslims have customarily divided the world into two parts: Dar al-Islam and Dar al-Harb. The literal meaning of Dar al- Islam is “House of Islam”; it is also sometimes translated as “House of Peace.” It refers to the regions and nations of the world governed by Islamic law. The literal meaning of Dar al-Harb is “House of War,” and refers to parts of the world not subject to Islamic law.

Much of Islam’s history is the story of how Muslims have sought to expand the House of Peace—an expansion often carried out by military conquest. Historian Karen Armstrong notes that expansionism was justified by “Muslim jurists...