The Muslims conquered the Sassanids, who conquered the Parthians, and the Byzantines, who descended from the Romans, and who conquered the Seleucids, the Attalids, the Antigonids and the Ptolemids, who were in turn descended from Alexander the Great, who conquered the Achaemenids, who conquered the Medes, etc.
Nobody has the moral ground, because every piece of land was conquered from somebody else if you look back far enough.