Pakistan is the second-largest Shia country in the world, with about 30 million population.
Also potentially, there are as many Shias in India as there are in Iraq.
Pakistan has the second-largest Sunni population in the world, while India has the second-largest Shia Muslim (Twelver) population.
A dispute over succession to Muhammad as a caliph of the Islamic community spread across various parts of the world, which led to the Battle of Jamal and Battle of Siffin.
The dispute intensified greatly after the Battle of Karbala, in which Hussein ibn Ali and his household were killed by the ruling Umayyad Caliph Yazid I, and the outcry for revenge divided the early Islamic community.
Shia theology has two concepts that define religion as a whole.
There are Roots of Religion (Usūl al-Dīn) and Branches of Religion (Furu al Din).
The overwhelming majority of that population lives between Pakistan and Lebanon.