Iraqi Shi'ite militia involvement in Syria's conflict exposes how rapidly the crisis has spiraled into a proxy war between Assad's main ally Shi'ite Iran and the Sunni Arab Gulf states supporting mostly Sunni rebels fighting the president. 

The conflict has already drawn in a stream of Sunni Islamist fighters from across the region attracted to the rebel cause, while on the other side Syrian rebels accuse Lebanon's Shi'ite Hezbollah of supporting Assad's troops on the ground. 

For Iraqi Shi'ites who follow Iran's Ayatollah Ali Khamenei, the uprising in Syria threatens Shi'ite influence and Iraqis fighting there say they see a duty to help Assad because of their loyalty to the Islamic Republic's highest authority.