This Shia Sunni rift is nothing.
It seems that itself the history of religion of Islam was much more ruthless than any Sunni/Shia/Safawi/ISIS.
Initially the Sharia Laws (dealing with Non-Muslims) were lenient. But in 9th year of Hijri, when Islam completely dominated, then Sharia Laws became ruthless. New orders came which were as under:
1) Only Jews and Christians (Ahle Kitab) are allowed to pay Jizya and save their lives by doing so.
2) But Kuffar were not even allowed to pay Jizya. They either had to accept Islam, or they would be slaughtered.
Here you can read the whole details.
https://islamqa.info/en/34770
So whoever refuses to enter Islam should be fought when the Muslims are able to fight, until they either enter Islam or pay the jizyah if they are among the people who may pay jizyah.
The kuffaar should be compelled to enter Islam if they are not people from whom the jizyah may be taken, because that will lead to their happiness and salvation in this world and in the Hereafter.
Obliging a person to adhere to the truth in which is guidance and happiness is better for him than falsehood. Just as a person may be forced to do the duty that he owes to other people even if that is by means of imprisonment or beating, so forcing the kaafirs to believe in Allaah alone and enter into the religion of Islam is more important and more essential, because this will lead to their happiness in this world and in the Hereafter. ....
For more details, go to the above mentioned link.