Let me tell you about wars in Islam as this is what I know.. I don't know about other nations' wars.

When the Prophet Muhammed received the message of God to guide ppl from paganism to monotheism he peacefully called his ppl to reject worshipping idols. He used logic with them, but the VIPs resisted and plotted to kill him not because their Gods told them to do so, but because they dreaded the ides of being treated in equality with their slaves and the poor in their community.

Others (namely the jews) were expecting a prophet to be sent in Arabia as the Torah told them, but they were hoping that he would be from among them. When they heard that he was chosen from among the Arabs they pledged to fight him.

This is how it began so the Prophet HAD to fight back!