What would we do though? Besides closing our borders I don't know if it would be legal to use ones religion as an immigration qualification.
Civil war though? In Europe they would have a hard time finding weapons I think unless they got enough people in the military/police etc. There is always the chance at a non violent take over through the ballot box if they had enough voters.
Brings up an interesting question here though, since we have the right to bear arms, that means if they had enough members they could arm themselves to do some damage in America.
Closing, or severely limiting immigration from certain regions is not without precedent. We did it with the Irish, the Italians and the Chinese. I'll have to check, but I don't think there's any reason we
can't limit immigration based on religious ideology. Any country has the right to impose immigration restrictions that best suit their culture. While the Constitution protects a
citizen's right to practice a given religion, there's nothing that says we have to admit people of any religion so they can practice it here. Some will find it morally reprehensible, but, given the ideological incompatibility of Islam with our culture and laws, I think it can be argued that we have every right to protect our way of life.
In the event of civil war I believe that, as with any war, the weapons will find the people. They always do.
The "nonviolent" takeover you speak of is happening right now in Europe, via immigration and a birthrate in excess of that of the native population. Civil war will break out when the native populace finds themselves subject to the laws of a religion they don't subscribe to, and they realize they've become an ethnic minority in their own country.
Here in the USA? I would say violence would break out sooner, rather than later. Due to the fact that such a large percentage of the population is armed, fully a third of the population. The equivalent of the largest standing army in the world.
Again, the question is: Do we allow this to happen here? Or do we allow the importation of our own demise as espoused by the current administration and liberal "Do Gooders", because Europe is doing it.