They already are here!
Figures vary, but there are probably no more than 5 million muslims in this country (don't count Calypso Louie's Nation of Islam folks. They've got their own ideology and aren't really muslim).
So, of that five million, half are going to be women and they aren't going to be a part of any army. Of the remaining 2.5 million, maybe a third are going to be too old or too young to be part of any army.
That leaves you with a potential pool of around 1.5 million military-age muslim men. Even if every single one of them picked up an AK-47, do you really think they could conquer half of the USA?