But why would groups within "the religion of peace" be attacking each other?
The Sunnis consider the Shiites heretics.
Roughly- After Mohammed died he was followed by several Caliphs the fourth one was Ali. When Ali died it split this group called Shiites away from the Sunnis.
The Sunnis continued with the Caliphs saying any descendant of any Caliph is a legitimate Caliph.
The Shiites felt only the descendants of the last Caliph Ali would be a legitimate Caliph. Over this they split away from the Sunnis.
The Sunnis stayed with the Caliphs and the Shiites went their own way with Ayotollas etc.
The Shiites only represent about 15% of the Muslims in the world today. Concentrated mostly in the Iran Iraq area.- Tom