> the Islamic conquests primarily have the distinction of being carried out by nations who if not foreigners themselves were driven by motives—the Islamic faith—entirely foreign to the invaded lands.
from a scripture point of view, they are both Abrahamic religions, use a book as the center of wisdom, buildings to house worship.. but let's set that huge likeness aside.
India, the Middle-East and Europe, all expanded at a certain time via tribal groups maturing into land-owning, army building principalities. The City-state and the male-heirarchy of arms, taxes and authority. The book contains the "King of Kings" .. this seems trivial to observe but it is not. Original tribal groups had vastly different social structures, religious beliefs, literacy rates but ultimately did not scale.
I propose that the Muslim expansion into Christian lands was polarized but similar military methods on both sides, all with the knowledge of Rome in the background. Contrary to what each saw when looking in the mirror each morning, they had a lot of similarities. Compare to trading, agricultural or other practices, who saw these Empire builders as the blood-thirsty ego cases that they were. Compare to the military conquest of South America by Christians much later.
Invoke "otherness" all you like, but there are layers to that, and more layers under that. Evolution ends quickly by the sword, and those empire builders, of all stripes, ended a lot of human social evolution exactly that way.