The fact that Muslims fight each other does not change the fact that Islam cannot co-exist, except uncomfortably, with Non-Islam. All you're saying is that Muslims cannot get along--with anyone. Is part of this the fact that they're human beings? Yeah, definitely, but culture and ideology matter, culture and ideology shape and dispose. When you begin with an all-powerful Deity whose actions are arbitrary, unquestionable, and all-controlling, your culture tends to take a certain path...
Which "Teuton invasion" are you referring to? We keep coming back to the fact that no one is fighting wars in the name of Jesus Christ any more. "Deus Vult" is history. Would that we could say that about the battle-cry of "Allah u akbar!"
It's not about whether the Muslims were "slightly less developed," it's about what their values were and are. There have been, and are, numerous "developed" cultures that none of us would have wanted to live in.