Excerpts from Books and Wikipedia
"Yazdegerd simply decreed that Zoroastrianism would be followed in all parts of his empire, including the Persian half of Armenia, which up until this point had remained Christian. . . . Like Marcian, Yazdegerd II had both political and theological motivations. He was devout in his faith, but the decree was motivated more by a desire to root out any sympathy with the eastern Romans—particularly in the less reliably loyal parts of his empire. The Armenians saw the decree both as an abridgment of their freedoms (which it was) and as religious persecution, and refused to give up their Christian faith. Yazdegerd reacted with force. In 451, the same year as the Council of Chalcedon, he marched into Armenia. . . . Ultimately the Persians were victorious over the smaller Armenian army; Yazdegerd II imprisoned and tortured the surviving leaders and reduced Armenia to a Persian territory, putting a new governor in charge of the country." [Bauer: Medieval World, p. 123]