China is the one to be most worried about. It has the biggest standing army, beating us out by almost 1 million more at 2.035 million, it has huge swaths of money and power, countless nukes, it has America's culture in the palm of its hand, it has the most grasp over us. EU is dirty and evil too, but they have enough of their own internal problems that I don't think they could pull together a decent enough force against us, and Canada is so weak in more ways than just the military that I don't think they pose much of a threat other than the fact that they are our border neighbors.
China is the one to be most worried about. It has the biggest standing army, beating us out by almost 1 million more at 2.035 million, it has huge swaths of money and power, countless nukes, it has America's culture in the palm of its hand, it has the most grasp over us. EU is dirty and evil too, but they have enough of their own internal problems that I don't think they could pull together a decent enough force against us, and Canada is so weak in more ways than just the military that I don't think they pose much of a threat other than the fact that they are our border neighbors.