Majorities of Americans are fair and just if they know the REAL truth, not the ones from the media. But there are some tiny bit aggressive ones. Here are some of the sceneraios you most likely encounter. Here are some answers that will touch their nerves really bad. But remember, ONLY use them against this kind of people.
Example 1: They just believe in their medias no matter what.
Ask them: Did you believe that IRAQ had massive destructive weapons when Bush started the War? Did you believe Saddam had connection with Bin Laden?

Response: Yes. (Of course yes, these are the excuses of Bush administration to ivade IRAQ)

Ask: Do you still believe now? (Background: After years of IRAQ war there have been no evidence that Saddam possessed massive destructive weapons or had plan to develope massive destructive weapons. And recently, pentagon quietly release the document that there was no evidence that Saddam had connection with terrorists.)

Now he/she will be in an very tough situation to answer. If yes, you go: You shouldn't cause goverment already said no connection. If No, you go: Why did you believe before and don't believe now? (Because the goverment lied. It was roughly estimated that Bush, Rasfield and Powell have iterated over a few hundrends times in public that they had solid evidences for the above accusations.)Then you go: I'm sorry man, your goverment lied to you. So could your medias.

Example 2: There is rumor that Han committed Genocide to Tibetan and killed 1.2 million Tibet during 20 years.
First show them the numbers of Tibet before 1959 and the numbers now. (Sorry I fogot it). Tell them Central goverment allow TIbet family to have up to three children and Han only one. Then the following is going to kill him/her. Refer to US history: Before Euoropean found US, it was estimated American Indians population at 80 Million. Now they are only 0.8 Million, thanks to Euroeapeans Genocide. (True Genocide here) Such records are easily availabel in US library by historicians.

Example 3: This has happened in Paris and may happen here too. Some may yell at you finally:"Go back to your home!'
You ask: Are you descendent of American Indian? (1 in a Million chance he/she is). Then go; I came to this land leagally. But you, your Great Great Grand parantes took it away from native American Indians illegally. I think you should go back to Europe!