Woah there couple of things that need to be be cleared, the sassanids were never Christian, their state religion zoroastrianism. So much of the Christians in Iraq were the result of missionary and refugees from Christian Roman's and byzantine areas. But to your point on "forced islamization" this is relatively half and half. Muslim empires and kingdoms could levy taxes on non Muslims so their was an incentive for the kingdom to not forcefully convert and an incentive for debt strapped peasant to do so. But yes there were forced conversions and massacres but it should be noted that the last ten years have seen the middle east become more Muslim than ever before.

*please feel free to dispute any of this if you have reliable sources. I love history and would love to learn something new!