Quite from https://www.quora.com/
Get rid of believing this fiction created by the most backwards western politicians and media that all conflicts and troubles between Muslims is summarized as a Shi'a vs. Sunni argument. It just does not apply in reality.
Examples are plenty but to point out a couple; Syria is a non-Shi'a regime backed and supported by Shi'a Iran. Azerbaijan, a prodominatly Shi'a country, is an ally of war-mongers in US and Israel against Iran. Sunni Saudi Arabia does not support Sunni Palestinian elected government of Gaza under Hamas.
The west can not be bothered to learn about an area, its history and how different people interact with others they don't share some basic beliefs with. The rapidly shortening attention span can only tolerate putting a couple of labels on 1.6 billion population of one religion in order to reduce it to a Cowboys versus Indians scenarios of Hollywood.
Such shortsightedness proved detrimental when all signs were ignored against the Saudi backed Al-Qaeda prior to September 11, 2001. Next time, the consequences may be much worse, as it has been for millions of Muslims already when Israel and other U.S. allies backed and equipped the "Syrian Liberation Army" that became the basis for creating ISIS.
Source https://www.quora.com/Why-is-ISIS-fighting-the-Kurds-if-both-are-Sunni
Interesting
This is from wiki "The first defections from the Syrian Army during the Syrian uprising may have occurred end of April 2011 when the army was sent into Daraa to quell ongoing protests. There were reports that some units refused to fire on protesters and had split from the army.[66] Video footage showed civilians helping defecting soldiers who had been shot for refusing orders.[67]" source https://en.wikipedia.org/wiki/Free_Syrian_Army