That is incorrect. The Islamic Empire, in the sense of a political entity governed by an Islamic warrior elite, was certainly spread through conquest as the Arabs took down the Byzantines and Persians, who were so busy fighting one another to exhaustion that they were unprepared for a fresh onslaught from a more dynamic group of people. But Islam itself could hardly have been spread by conquest; the conquering armies were much, much too small to enforce such a young faith over the very well-established religions of Byzantium and Persia. In fact, the newly subject populations, being "Peoples of the Book," were allowed to keep their respective faiths subject only to certain restrictions (like paying special taxes and being unable to build a church or temple higher than a mosque). Only gradually did these populations convert to Islam, and then only because the new faith seems to have offered them a message lacking in their established traditions. Significantly, Islam was spread to sub-Saharan Africa and Indonesia (where more Muslims live than anywhere else) via commercial contacts, not warfare: people chose this religion willingly more often than they did not.
If you want to hear about warfare and forced conversions, then check out Christianity, which was confined to the urban centers of the Mediterranean basin for centuries until Germanic warrior kings, setting up their own polities on the ruins of the Roman Empire, started forcing their subjects to convert (a task made easier, again, by the relative intellectual sophistication of Christianity as compared to their own pagan traditions). Often these warlords themselves were only converted spontaneously, in the heat of a losing battle, and considered Christianity a kind of magic that would help them keep winning their wars.
You point about the Islamic armies not being able to force Islam on the populace would make sense to those ignorant of the Muslim practice of Dhiminism, or forcing those who are not Muslim into the edges of society by imposing heavy taxes on the "Dhiminis", or non muslims, and otherwise making them into third-class citizens of Dar al-Islam. Those populations "gradually converted" in order to escape the oppression and burdens they bore as non Muslims, not because of any lack in their previous faiths.
As for your statement about warfare and Christians, you totally ignored my earlier statement about Christianity not embracing violence until after it became the religion of the Roman empire. That is a fact of history you can't get around. The Christians of later eras that you mention were converts to a religious institution which had already turned it's back on the nonviolent message of the early church. Your rebuttal hardly overturns my observation that Islam embraced violence from its very beginings rather than learned it from "Christians", as the person I was rebutting implied.