The apostle started war against other religions for these reasons:
1- As for non heavenly religions as Buddhism, Magianism and others, because those religions were invented by man not revealed by heavenly divine revelation, so their orders and rules were confused a
nd they were not suitable for people at different places or times, mature minds also couldn't accept what they order.
Whereas the Jewish and Christians people who followed the truth and kept to it, they believed in him and followed him as mentioned in those religions and recommended by their apostles.
But those who sought benefits of this life and priesthood or clerical positions changed and distorted their religions to suit their desires, they also told their followers untrue sayings about Islam and its prophet, different from the facts they know and mentioned in their religions.
2- As the apostle was sent by the truth from Allah to all people as mentioned in the Koran:
“We have not sent you but as a universal (Messenger) to people, giving them glad tidings, and warning them (against sin),”
Surat Saba' Verse 28.
So he had to make the facts of religion illustrated by Allah clear to all people and everyone prevent him from expounding this he should dislodge him so as to be able to do the mission instructed by Allah.
3- If we returned to the facts of history, we see that the apostle didn't declare war against such people but they incited their kings and their followers to collect armies to fight him, their bishops and rabbis carried out the morale war to mislead beat his religion, so he (peace be upon him) was just defending himself and declaring the orders, morals and principles he got from his Lord and we see the time proved and still proving that they are the most useful for mankind at all times and everywhere, humanity need this badly now, that's why the English writer Bernard Shaw said after the second world war in which more than 30 million people were killed: If Mohammed was among us now, he would solve all the problems of humanity
as if he is having a cup of coffee