When Fatima Pashaei, a Shiite Muslim of Iranian descent, and Atif Qarni, a Sunni Muslim whose parents emigrated to the U.S. from Pakistan, married six years ago, it was a joyous event. "Our parents were just happy we were marrying Muslims. Everything else is sort of secondary," said Pashaei, 24, still married to Qarni and living in Manassas, Va., with their 3-year-old son. Divisions have widened worldwide since the golden dome of the Al-Askariya Mosque in Samarra, Iraq, was blown up Feb. 22. Two descendants of Islam's Prophet Muhammad are buried there, making it one of Shiite (also called Shia)...