What was the power of Iran based on in the first centuries of the Middle Ages?

This is the era of the Sassanid dynasty. The states in those years were agrarian, therefore, power was built on control over a large number of fertile lands.

Also important is control over the trade routes that connected India and China in the East and Byzantium and other countries of Europe in the West.

And one more indicator – successful wars, control over deposits of metals, gold and silver.