What are the origins of Shia and Sunni division?

Can anyone tell me about the origins of Shia and Sunni division? Why were two different groups of Muslims created? Allah didn't send us to live divided. No verse of Quran or Hadith suggest that as far as I know. Both of the groups are driven by great scholars. They know Quran and Hadith very well still they are leading to something that Allah does not like? Is there any political involvement?