The Sassanids of Persia under Ardashir I destroyed the Parthian Empire and conquered the region in 224 AD. During the 240s and 250's AD, the Sassanids gradually conquered the independent states, culminating with Assur in 256 AD. The region was thus a province of the Sassanid Empire for over four centuries, and became the frontier and battle ground between the Sassanid Empire and Byzantine Empire, with both empires weakening each other, paving the way for the Arab-Muslim conquest of Persia in the mid-7th century.