To many Americans, the people of the Middle East appear culturally and ethnically similar. Yet those outward similarities mask profound religious, historical, and political divisions that have shaped the region for more than fourteen centuries. These divisions are rooted in the birthplace of Judaism, Christianity, and Islam.
For decades, much of the world's attention has focused on conflict between Israel and its Arab neighbors or between Islam and the West. In recent weeks, however, the center of gravity has shifted back toward one of the region's oldest rivalries—the struggle between Sunni and Shia Islam.
I believe there are two reasons for this shift.
First, Iran's military capabilities have been significantly degraded. Second, Iran's leadership must now decide which adversaries pose the greatest long-term threat to its revolutionary ambitions.
A Fourteen-Century Divide
The Sunni-Shia split began after the death of the Prophet Muhammad in A.D. 632 over who should lead the Muslim community.
Sunni Muslims, who represent approximately 85 to 90 percent of the world's Muslims, believe the leader, or Caliph, should be selected by consensus among the Muslim community and should follow the traditions (Sunnah) established by Muhammad and his earliest companions.
Shia Muslims, who represent roughly 10 to 15 percent of Muslims worldwide, believe leadership

