Why has the US only attacked countries who do not have militaries that stack up against theirs the last 40yrs?

Is the US afraid of taking on a superpower? The US military lost the Vietnam War and two wars in the Middle East? Is their military really the best in the world? China is presently violating the human rights of some of their citizens yet the US isn't going after them like they did Saddam? What gives? Why isn't the US trying to convert Russia to democracy? Or China? or North Korea? It appears that the US government only bothers countries who have no tanks, jet-fighters and the like. have you noticed this trend the past 40 years or so? Now, I know it's not the soldiers fault as they are doing what they are told to do. The elected officials who are supposed to work on the behalf of the people are to blame.