Power is always relative. The Roman Empire had lost some territory to the Persians, but this did not hugely reduce its resources. It remained the big beast in the jungle. For the Persians however, acquiring some new provinces enhanced their capabilities considerably over where they had been before the peace treaty. They were still at a severe disadvantage in an all out fight with the Romans, but they posed a much bigger threat than they used to. This meant the forces deployed to defend against them had to be larger and they had to be treated with a greater degree of diplomatic finesse.
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