Your question sounds deceivingly simple, but the answer is anything but.
Militarily, America seems to be the most powerful national force, by far. Economically, America still seems the strongest.
The policies of the United States have dumbed down Americans over the last few decades. Look how easily we were tricked by 9/11. Americans have been coddled by their government and Corporate Party media kool-aid to the point of Learned Helplessness. The media has also been effective in magnifying the effects of Normalcy Bias so that people's disbelief that America is crumbling makes it appear far stronger than it really is. It took living outside the States for 5 years for me to wake up to this.
Corporations have blurred the lines on what is a nation or country. The super elite have secretly moved their agenda forward for decades, first creating the European Union, and then establishing the foundation of the North American Union. We see this in the reticence of the federal government to tighten up its border with Mexico. There is already talk in economic programs on TV for the establishment of the Amero to replace the dollar.
If sovereignty is a measure of nationhood, then America has lost its status as a nation. First of all, massive debt makes us slaves to the banks, especially to the very private Federal Reserve central bank. Evil loves to operate in private and in secrecy. That describes the Fed and its aim to remain secret. Dr. Ron Paul's recent win in the House with his "Audit the Fed" bill made the Fed very anxious. Its owners, the Rockefellers and Rothschilds are no doubt stepping up measures to stop the same measure in the Senate. If anyone loves what America once stood for, they should flood Majority Leader Harry Reid's phone and email with requests to bring it to a vote, and they should demand from both their senators to support the Senate measure.
America has been for some time a puppet state of the Corporations and their owners. The war in Iraq had nothing to do with WMDs and Al Qaeda. It had everything to do with Corporate profits -- like Dick Cheney's cash cow, Halliburton. The Bush administration was one big conflict of interest. And 9/11 was their pet project. It makes me sick.