Western Armenia had been under Byzantine control since the partition of the Kingdom of Armenia in AD 387, while Eastern Armenia had been under the occupation of the Sassanid Empire starting 428. Regardless of religious disputes, many Armenians became successful in the Byzantine Empire and occupied key positions. In Sassanid-occupied Armenia, the people struggled to preserve their Christian religion. This struggle reached its culmination in the Battle of Avarayr. Although the battle was a military defeat, Vartan Mamigonian's successor, Vahan, succeeded to force the Persians to grant religious freedom to the Christian Armenians in the Nvarsak Treaty of 484.