China is now what the US was in the 1930s. In 1930, the US was not a threat to Germany or Japan. But, we all know what happened. Economic power can easily become military power when a nation is properly motivated...and China is motivated.
Right now, I would rank the US #1 and China #2 in terms of ability to dominate/influence the world.
But, I think those two countries will trade places in the next 20-40 years, and the next 150 years after that will be dominated by China. Right now, they are keying on stealing, repoducing, and replacing all the major industries that the US has. Their economic power will inevitably translate to military power. Right now, they cannot project power like we can. But, I would still argue that they are the biggest long term threat to replacing the United States as the leading superpower. Do you think there is another that is a great threat in that way? I can't think of a single one.