Birth Rates. To the tune of 8:1 in Europe. They won't need guns. They won't need to impose sharia on anyone. The demographics will shift totally in a couple of generations and then everyone will choose sharia.
I doubt it's that quick or easy of a transition, but I was going to essentially say the same thing in response to huff's post. It's not a matter of whether or not the "forces of Islam" are powerful enough to takeover Europe. That's a first and second millennium model. Things don't necessarily work like that so much anymore in this era of high tech weaponry. Rather, Islam can perform a de fact "takeover" of Europe through more perfidious means, like demographic change, as you point out.
The key to assimilation is and has always been diluting the potential power of any particular group in the total immigrant pool by diversifying their numbers and by allowing immigrants from numerous different regions/cultures. This is how the US, and Europe to a lesser extent in the past, was able to properly assimilate immigrants. No one immigrant group ever gained enough numbers and/or power to create a resistance bloc to the dominate, established native culture.
The problem now, however, is that immigration to Europe and the US is the least diverse it has ever been, in both cases. Europe's mainline of immigration streams from the Islamic world, while the US's largely comes from Latin America, particularly Mexico. In both cases, potential problems lurk down the road, simply because of demographics (natives aren't spitting out babies while the incomers are) and the lack of a diverse immigrant pool. This means that, in each case (Europe and US), nations may possibly have to confront disproportionately powerful and influential immigrant power blocs. While, it's not a given, this presents possibilities like secessionist movements, ethnic enclaves, ghettosization, immigrant-dominated political opposition (like a hypothetical Islamic Party of France), alternative cultural models competing with the previously dominant one, etc.
Luckily, for the US, Mexicans (which are our biggest potential immigration problem, although not necessarily) share much more in common with us than Muslims do with Europeans.
