There are two superpowers in the world today. The United States of America, and anti-Americanism. Anti-Americanism is very powerful, as it is the sordid glue that holds the UN, fifth-column Americans, Euro-socialists, the world's fashion elite, and terrorists together. It is the invisible force that forces the US to withstand massive double standards that have been there for so long that they are taken for granted.
Interestingly, with the exception of terrorists, such individuals go to great lengths to conceal their anti-Americanism, pretending to stand for 'nonviolence', 'peace', 'equality', the 'world community', etc. This begs the question of why they don't feel comfortable with declaring their dislike for the US. Maybe it is due to the guilt of knowing they are being unfair, or guilt that they are opposing a force of liberty and prosperity in the quest to be fashionable. Maybe it is all this plus juvenile envy of success and power. In any case, having a strong dislike for America, yet not having the integrity to be honest about one's true feelings, makes such a person easy to defeat through skillful debate.
There are many ways to do this. Two examples are below.
Option 1 :
While many who say this are merely fashion-parroting sheep rather than committed anti-Americans, if someone you believe to be a genuine anti-American says they oppose the Iraq War because "there were no WMDs" or "Bush lied about WMDs", then you can merely ask :
"So if WMDs were found, would you support the war?"
They can either answer "no", to which you can say "So why do you present the absense of WMDs as your primary objection to the war, if you still would have opposed it anyway? That appears rather phony on your part."
Or they can answer "yes", to which you can ask them "But Iran and North Korea are openly admitting to the pursuit of nuclear weapons, and are threatening to use them. By your logic, invading them is fully justified, is it not?"
They have thus revealed that they merely avoid taking difficult decisions, in order to criticize from hindsight and mask their anti-Americanism in pseudowisdom. Either way, they are trapped. This is so simple, yet very effective. In reality, they oppose any action by the US because they oppose the very ideals of the US. Yet, they are too ashamed to admit it, and so hide behind phony guises.
Option 2 :
If you are the one who wants to initiate the debate, you can openly declare that "I feel that America, despite many flaws, has done more to benefit humanity than any other nation existing in the world today." If your opponent is a secretive anti-American, they may react with sputtering outrage (blowing their cover). They will point out various acts of evil that America has done (some true, some imagined), but it will become apparent that they are judging America to some utopian standard, rather than in relation to other countries existing in the world today. To this you can merely reply :
"Which country do you feel has done more for humanity than the US?"
"If an Asteroid were on a collision course with the Earth (never mind which country's instruments detected the asteroid), which country would be expected to take the lead in an effort to destroy or deflect the asteroid?"
In either case, the anti-American will be cornered, and seek to change the subject, or become visibly annoyed.
Expose their anti-Americanism, and you will gain a greater understanding of this shadowy second superpower.
Prediction : Anti-Americanism will become more pervasive and tightly unified, with many Western anti-Americans moving from merely condoning the activities terrorist groups such as Al-Qaeda, to engaging in covert yet active support of such groups.
Update : Read the comments section. Many fashion-parroting anti-Americans have come by, have been posed simple questions about their principles, and have fled without answering, for fear of revealing their anti-Americanism. It is quite entertaining.