What is the main reason behind the conflict between the shiites and the sunnis?

The religious conflict between Sunnis and Shiites has its origin in the succession of Muhammad as the main prophet of Islam:

-Shiia Islam is the second largest branch within Islam. It is estimated that around 30% of all Muslims follow Shiia tradition, while about 70% belong to Sunni Islam.   Shiia means "a successor to Ali". Ali ibn Abi Talib was Prophet Muhammad's cousin and son-in-law. He was the fourth Caliph, the Prophet's successor as Muslim leader, from 656 to 661. Shiia Muslims, also called Shiites, believe Ali and his family were chosen by God as Imams, political-religious leaders, for the entire Islamic community. Prophet Muhammad declared this during the "Ghadir Khumm" incident. Here he asked all Muslims who accepted him as their leader, also to accept Ali as their leader.   Shiites constitute the majority of Muslims in Iran, Azerbaijan, Iraq, Bahrain, Lebanon, Pakistan, India, Yemen and Turkey.

-Sunni Islam is the largest branch in Islam, covering approximately 70% of the world's over 1.6 billion Muslims. Sunnah means the pattern of the Prophet through whom the law of God is given.  

Like Shiism, Sunnism arose from the struggle for the lawful succession of Muhammad, which began after his death in the year 632. The Sunnis assume that Muhammad did not designate a successor during his lifetime, nor did he indicate how a successor had to be determined. The battle was between supporters of Muhammad's cousin and son-in-law Ali and those of the Caliphs. The Sunnis are followers of the latter. The first caliph was Muhammad's father-in-law Abu Bakr.

answer; i believe the correct answer is (crops);

no one should discriminated against because he or she is not from a minority group