what caused islam to split into the sunni and shia branches

Question

blazeryder · Answer

Take a look at this: http://webspace.ship.edu/cgboer/sunnispelletes.html

cecil_the_weasel · Answer

It has to do with a disagreement about who was to succeed Mohammad when he died. 

I would type more but BlazeRyder already suggested the site I was about to quote. You should not have to look far to find your answer. :)

otherworldly · Answer

The sunni branch is that of the actions that are recommended because Prophet Muhammed did them and in the Muslim's holy book ( the quran), it tells the Muslims to follow what Muhammed did (called his actions sunni) 
The shai branch  are the people who follow the hadiths and not-so-much the sunni. The hadiths are just sayings that the prophet said and the people recorded it for the future generations to follow and obey. Their god ( Allah ) told all Muslims to follow what the prophet did for he is the best role model and the last teacher ( he was the last prophet)

otherworldly · Answer

in later generations the Muslims started to split into two branched because they felt one or the other was more important and obligatory.