In The Divide (2011), I am confused as to who nukes the USA? Was it the Arabs? Maybe the Koreans?
That question is never answered in the film, and I believe it is a deliberate strategy by the filmmakers so that the focus is on the group dynamics that ensue, and not all about pointing blame at any one country or faction.
The fact that the American 'men in white' seemed to be extremely well prepared for this event does seem to imply that it was a homegrown act - but this is purely speculative.
If you freeze frame the shot of the ID badge when the character Josh gets caught looking at the girl who was previously taken from the shelter you find your answer. The picture on the ID is that of an Asian man, but the tiny print above the bar code at the bottom of the badge reads: Department of Containment and Preservation. It was an inside job. The city was nuked to contain something that can apparently be traced in the glands of prepubescent kids (hence why the soldiers look down the girls throat with a flashlight prior to taking her). Sort of a "sacrifice the few for the good of the many" thing but on an appalling scale.
In a conversation between Delvin and Eva (while they were trying to make contact using the radio), it was mentioned by either of them that they heard soldiers (in hazmat suits) speaking in english... with local accents.
It was a purely deliberate act by the government, perhaps for the purpose of keeping the world's population in check. The kidnapping of Marilyn's daughter and her being kept under stasis, is testament that they are also practising natural selection. The movie presents to us a glimpse of how a shift into a new world order (perhaps under a one world government) might have been put into motion.