Assalamu Alaykum,
Brother, why did Abu Bakr conquer Iran? Didnt he force his religion on them? Also why is the Prophet's letter to Oman threatening?
"“In the name of Allah, the Beneficent, the Merciful. From Mohammed, the messenger of Allah, to Jaifar and Abd, sons of Al Julanda. Peace is upon him who follows the guidance. I am calling both of you, in the name of Islam. You will be safe if you submit to Islam.
I am the Messenger of Allah to all people warn all living that Islam will prevail. I hope you will accept Islam, but if you do not, then you will lose your country, and my horsemen will invade your territory and my prophecy will dominate your country”."