"The 50 years after Muhammad's death witnessed a succession of caliphs who attempted to carry the Prophet's message forward." (Rogerson)When Muhammad died of an illness, disagreement and conflict broke out due to the wide inquiring of who should succeed Muhammad as head of the Muslim population. Two people were nominated, the first being Abu Bakr, Muhammad's friend and associate, and the second Ali ibn Abi Talib, Muhammad's cousin and son-in-law. Followers soon joined each individual. As a result, the two Muslim sects were formed. These sects were known as, and continue to be known as, Shia and Sunni. These events set the precedent for resulting conflicts within Islam.
In modern times, Shiite and Sunni Muslims continue to hold on to their disdain for each other. To this day, there are still differences in practices, traditions, customs, and beliefs, among others. Roughly, the vast majority of Muslims are Sunni; estimated to be around 85%, leaving the Shia/Shiite Muslims with 15% of the Muslim population.
Within Sunni beliefs, Abu Bakr was the rightful and sole succesor of furthering the life of Islam and Muhammad's teachings. The Sunnis also follow the rightly guided caliphs, which is known as Rashidun. These rightly guided caliphs are the four caliphs, or successors, after Muhammed. On the contrary, Muslims of the Shia sect hold to the belief that Muhammed's cousin and son-in-law, Ali ibn Abi Talib (or Mhammad Ali), was the divinely and specifically chosen successor of Muhammad.
Rogerson, Barnaby. The Heirs of Muhammad : Islam's First Century and the Origins of the Sunni-shia Split. London: Overlook Press, 2007.
Khan, Arshad. Islam, Muslims, and America: Understanding the Basis of their Conflict. Algora publishing.