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.

freeborn women in ancient rome were citizens (cives), but could not vote or hold political office. because of their limited public role, women are named less frequently than men by roman historians. but while roman women held no direct political power, those from wealthy or powerful families could and did exert influence through private negotiations. exceptional women who left an undeniable mark on history range from lucretia and claudia quinta, whose stories took on mythic significance; fierce republican-era women such as cornelia, mother of the gracchi, and fulvia, who commanded an army and issued coins bearing her image; women of the julio-claudian dynasty, most prominently livia (58 bc-ad 29), who contributed to the formation of imperial mores; and the empress helena (c.250–330 ad), a driving force in promoting christianity.

women in the ancient greek world had few rights in comparison to male citizens. unable to vote, own land, or inherit, a woman’s place was in the home and her purpose in life was the rearing of children. this, though, is a general description, and when considering the role of women in ancient greece one should remember that information regarding specific city-states is often lacking, is almost always from male authors, and only in athens can their status and role be described in any great detail. neither are we sure of the practical and everyday application of the rules and laws that have survived from antiquity. we do know that spartan women were treated somewhat differently than in other states. for example, they had to do physical training like men, were permitted to own land, and could drink wine.

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