Remarks on the Sunni-Shia division in Islam
The division of Islam into Shia and Sunni branches from the mid-seventh century was more due to the political factors than with the fundamentals of the faith because they were the same for all people including the power elites. Obviously, two rivals engaged in a struggle to gain the upper hand in a political race cannot win unless they strike some compromise and avoid the conflict. This was possible between the rival claimants, Ali bin Abi Taleb and Muwaiya bin Abi Sufyan, but did not happen in the early phase of the growing polarization that was taking place in the Muslim community.
One puritan group, the Kharijites, saw the developments with apprehension for the new faith and the Muslim community, which by now was large and was rapidly spreading in many regions of the world. The Kharijite solution to stem the tide of power-politics that was damaging the new nation was a radical one: liquidate the rival claimants to the Caliphate and save the faith and the Caliphate. Despite what they did, the problem did not vanish.
Now the Sunni and Shia forms of thought about the office of the Caliph started to diverge. Afterward, even the theological differences started to grow. Inevitably, the Sunni and Shia doctrinal differences became more pronounced and the different schools of jurisprudence put their stamp on the growing disparity between the two groups. Therefore, what started as a political factor eventually developed into two rival sects within Islam.
What sort of relationship emerged between the two branches when Islam became a world religion and Islamic Empire grew in size and power can be briefly put this way: The Sunni Islam became dominant but Shias were not victimized. The relationships were mostly cordial and one of toleration and mutual respect.
The intolerance towards Shia and the victimization of Shias in countries like Pakistan in these times is a tragic story of a faith that has been hijacked by some fanatic ignorant people in the name of their brand of 'puritan' theology. But these criminals and assassins are a right-wing fringe element within Pakistan. The great majority of ordinary Sunni Muslims has nothing against Shia Muslims and vice versa. Both of them look upon each other as brothers and accept each other's right to follow Islam according to their traditions and customs.
But in Iraq under President G.W. Bush American invaders and occupiers of the land fanned the sectarian divide, resulting in the bloodshed of Iraqis. The suppression of the Shias in Saudi Arabia is also a manifestation of how the Wahabbi rulers impose their morbid religious cult in violation of the fundamental principle of the freedom of religion which is followed in the civilized world.