There is a difference between fighting back, and initiating wars. Muhammad initiates wars. The Muslims after him, lead by people whose life we should emulate (Abu-Bakr, Umar etc) and generals who were friends of Muhammad (Khalid ibn al Walid for example) were into constant wars, first with Muhammad’s own tribe, then with the other Arab tribes, and finally with the Romans and Persians. These were not defensive wars, it was the Muslims who initiated them, with tens of thousands of people being killed of them.You are correct. Buddha and Jesus (by and large) were pacifists, they didn't believe in fighting back. Do you know what happens to people who don't fight back? This happens:
https://en.m.wikipedia.org/wiki/Moriori
In the words of the late Malcom X
"Our religion teaches us to be intelligent. Be peaceful, be courteous, obey the law, respect everyone; but if someone puts his hand on you, send him to the cemetery."
It's a base human instinct to subjugate those who are weaker. Why would you be against a message telling people to hold onto their rights and fight their oppression.
With whomever they could, it was convert to Muslims and accept Muhammad as overlord (or later the khalif), pay the tax and accept Muhammad/khalif as overlord, or get killed.
Let’s be fair, it is a quite similar treatment to what ISIS gave to the ‘nonbelievers’ a few years ago. Or what Genghis Khan and Timur gave to their enemies (tho Genghis didn’t care about changing their religions, it was pay tax or die), or Alexander the Great gave his enemies. Sure, it was what the other leaders of that time did and it was perfectly acceptable. But it would look ridiculous if someone would start saying that Genghis Khan was a symbol of peace, preached peace and should be emulated.