Oopps, didn't see the U.S. on that list until after I voted. Of course we are the biggest threat to ourselves as we continually tolerate the behavior and massive population growth of foreigners and people of other races at the expense of our own. Only we can stop the rotting of the U.S. from within. No nation can or will be us by force at this point so the only real threat to the U.S. is our own policies.
Of the other choices Mexico is more responsible than any other country for the rotting from within that the US is currently experiencing. I equate it to the gradual fall of the Roman Empire and the rotting from within that took place during that time period. The Roman rotting took place win they lost control of their borders (perviously strictly controlled) and other peoples, who bore little loyalty to Rome, mercenaried into their military and in an attempt to contain an outstretched empire.
At the same time they made many others far away lands full Roman citizens - perviously a rather elite status. Soon being Roman meant very little and the Roman people lost the will to maintain the empire. That in a nutshell is what happened to perhaps the world's greatest empire over about a thousand years and that is what is happening to the current world's greatest empire in just about 100 years time.
WS, I'm surprised you didn't have Israel on the list. But maybe it is best not to get into that discussion as it tends to have a life of its own.