Prophets

The Prophet’s letters to rulers, was it declaration of wars?

Assalam u Alaykum. During a religious debate a very serious point was brought up by my colleague.During the time of Prophet Muhammad (peace be upon him) and the establishment of Islam over the Asia and Europe, letters were sent out by the Prophet to leaders of various lands. These encouraged them to take up Islam as it was a better way of life for them. My colleague states that after intial polite letters further incentives were sent to accept Islam. Thereafter what could be interpreted as threatening letters were sent therein stating accept Islam or we will wage a war against you. Is this true?An example given was when the companion Umar was told to go to Persia (Iran) and destroy thousands of books in the libraries as only Islam and the Quran was all that they needed. My colleague claimed Islam eradicated thier culture and way of life which they were more than content with. They were told the Quran is all that they need.Surely education and knowledge via books and libraries is fundamental in Islam as were are here but to acquire knowledge and spread that knowledge of the beautiful preachings and way of life of Islam.If this is factual surely Islam is about giving the message of Islam to others and then letting them decide if they want to accept it. Islam is a religion of peace. Why would the Prophet order war against a nation purely as a consequence of thier King not accepting Islam?If this is not factual please explain why the Prophet did what he did in the context of that time. Maybe the Persian King contributed to this reaction?Thank you for your efforts. May Allah forgive us all and grant us eternal paradise.