Here is so called Official Position of shias [shiites] is that these conquests were sharai’ wa sahih [means sahih according to sharia] and legitimate.
Q) conquest of Iran took place at the time of Caliph Umar. Was this conquest legitimate or not ?
A) Yes, it’s legal and right. Thats what “Fotooh AL-Sham and others” said, and it comes under the title called “The Lethal Wars”, and that was after the Persian and the Roman countries showed threat toward the new religion and both countries weren’t making any progress in it.