Who do you see as America's biggest threat in the years and decades to come?

[Verily]

Hard to say.

China has the potential to be an irritant bordering on a threat as long as the current government is in power. If anything ever topples that government, which is entirely possible within a twenty year or so time frame, China has the potential to become a major threat, and certainly such instability in China would cause problems worldwide.

Russia seems devoted to being annoying, but Russia is no threat in any actual sense of the word. It's just full of corruption, incompetence, and leaders with egos the size of Mercury.

The EU has the potential to be an economic rival, but not a threat.

All other nations are either too small, too friendly, or too economically backward or else otherwise unimportant.

I don't really see a major threat, as in a military threat, to the United States in the medium-term (meaning within the next few decades). Then again, I will tell you that the US has not been seriously threatened since the 1980s, so you understand my usage of the word "threat".