Why did prophet Muhammad threaten the king of Oman to accept Islam?

The letter of Prophet Muhammad to the king of Oman. >From Muhammad, the messenger of God, 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 peace. You will be safe if you submit to Islam. > >I am the Messenger of God 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**. > >Seal: Muhammad, the Prophet of God Why did prophet Muhammad threaten the king of Oman(*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*). There should be no force convertion to Islam. But by threatening that your kingdom will be conquered and you will lose your kingdom if you reject Islam, is force. And why would prophet say this in first place, I mean why did he threaten the king.