It is one of the great ironies that the United States, a land mass protected by two broad oceans while also benefitting from the world's largest economy and most powerful military, persists in viewing itself as a potential victim, vulnerable and surrounded by enemies. In reality, there are only two significant potential threats to the U.S. The first consists of the only two non-friendly countries – Russia and China – that have nuclear weapons and delivery systems that could hit the North American continent and the second is the somewhat more amorphous danger represented by international terrorism.
And even given that, I would have to qualify the nature of the threats. Russia and China are best described as adversaries or competitors rather than enemies as they have compelling interests to avoid war, even if Washington is doing its best to turn them hostile. Neither has anything to gain and much to lose by escalating a minor conflict into something that might well start World War 3.