They could definitely make things very dicey for a while, maybe even a few months, with some kind of guerrilla-style warfare in this country. Short of a top tier having WMD I have a hard time picturing an extended engagement - their C&C and supply chain would be long and very dicey (not to mention the problem of recruiting or importing new fighters since they would have little if any support from the population). Also, once the general population realized what's up there are (besides the police and the military) lots of good ol' boys with guns who would be only too happy to take some of them out. I suspect that it would be pretty much of a suicide mission ... not that it couldn't get real messy before they were all exterminated, but I think the final outcome wouldn't be in much doubt.
Add in WMD and the situation changes dramatically though...