What country (or countries) is a DIRECT threat to the United States?

I mean what country (or group if you prefer) could bring the entire United States to its knees and can wipe America and its’ way of life off the map; I don’ t mean one that might plant a bomb or lob a missle and blow up a few Americans or even a few thousand.

It might help if you could give the details of the one scenario you could forsee that would bring about the destruction of the United States.