Oh contrare, the US military is so powerful that it could take on the entire world and have a good chance at winning. Britain, although controlling the seas, could not best germany nor France on a land battle. Britain's economy, although dominant, does not hold the hegemony of the US economy. It never has had the cultural power that the US has controlled for the last 60 years.
Economically, the US is the largest economy ever in terms of percentage of world wealth controlled. Politically its alliances span nearly the entire globe. Economically, it is most often the largest trading partner to most of the countries in the world. Economically, its economic downturn or upturn determines the entire world economy. Militarily it can strike any target in the world with precision and deadly force. It could put the entire world trade on hiatus with its undoubtedly dominant navy. Not even the British could come close to that.
Politically, economically, and culturally, the US affects every corner of civilization in the world. The current world culture is modeled after American culture. Many musicians and actors move to the US once they hit it big. Economically, the US is the bigger by triple digits compared to the worlds number two. American politics is often world politics.
In the words of MW2 "what happens here matters over there". Very true words indeed.
Now this is pe*is-measuring but you started it... Feel the full fury of my nationalistic rant!

(although I know i will likely get pummeled because of this post, it is quite true...)