I don't think the letter is genuine. What is the Islamic source for this quotation?
Also, the Persians were invading Arab lands and enslaving its people. Plus, Allah Almighty commanded the Muslims to spread Islam. So Islam was spread. What is your objection here? It's not like the pagans were all about peace. Bringing GOD Almighty's Divine Truth and Mercy and Justice to mankind sometimes has to come through war.
Take care,
Osama