Simple. They merely displace the rest of us with their higher birth rates. Their increased demands on our social services will speed up the economic collapse. In the meantime, they partition our nation in an alliance with the left. (That way the left doesn't have to do the dirty work of fighting us.) It fits in perfectly with the way they do things.
They've only made one miscalculation: They think we'll display some form of compassion while defending the remnants of the US, thereby leaving some of them alive to continue their quest. They just don't know us very well.
To hell with bayonets, keep yer .50's even if they say it's a sin against society.