I don't know that I like the word "foe" in this very interesting question.
China seems like the biggest rival now. The rivals do seem to come and go. Germany and Japan were going to eat us up in 80s, then the united Europe was to be the new thing. Now comes China.
I'd have to say that our biggest problems are a political system that seems to be failing to seriously address, let alone solve, prominent issues during our election discourse. We give office holders mandates to allow/not allow gay marriage, starve/save the elderly, protect/attack the illusory "death tax" that most people think they will pay even though they won't, and many other issues that excite the heart and not the mind.
If I had to name this geopolitical foe, I'd call it Idiot World. It's influence pervades like a disease without consideration of party, religion, ethnicity, or, surprisingly, educational level. Idiot World has always been among us and in us all at one time or another, hopefully it only seems that its influence is peaking. It's the foe that could undo us.